What does battle icons (buffs and debuffs) mean?

You could read what every effect does by tapping on it during battle.

Check this article Positive and Negative Status Effects Explained. Quote from it (in case the link goes down):

Buffs:

• Critical Chance Up: This buff increases the Physical and Special Critical Rating by 25%.
• Defense Up: Increases the Armor and Resistance of a Hero by 50%.
• Evasion Up: Adds 10% to the dodge stat which governs the chance of avoiding any physical type damage entirely.
• Foresight: Grants a Hero 100% Evasion chance against one attack and then it expires.
• Health Up: Increases the maximum health of a character based on the percentage gain in the ability it generated from.
• Offense Up: Grants a bonus of 50% to Physical and Special damage stats.
• Potency Up: High tenacity characters tend to resist debuffs often, so this buff increases the chance to apply detrimental or negative effects.
• Retribution: Provides the character Counterattack.
• Speed Up: Grants an ally +25% speed providing faster refilling turn meters for more actions per battle.
• Stealth: Stealthed characters are not directly targetable unless only stealthed heroes are remaining.
• Taunt: Characters with taunt will force all opponents to target them until the effect expires.

Negative Effects or Debuffs

• Ability Block: Prevents a hero from using their special abilities. Carefully time applying this debuff so it’s not wasted if the enemy’s special is on cooldown.
• Buff Immunity: Prevents a hero from receiving positive effects.
• Continuous Damage: This is a Damage over Time (DoT) effect that reduces health at the beginning of the character’s turn.
• Defense Down: Decreases the Armor and Resistance of a Hero by 50%.
• Expose: A character takes damage equal to 20% of their max health the next time damaged.
• Healing Immunity: Heroes effected by this cannot be healed.
• Offense Down: Reduces Physical and Special damage stats by 50%.
• Speed Down: Slows the turn meter down by 25% which reduces the amount of actions a hero can take during battle.
• Stun: Stunned characters will miss their next turn.
• Bomb: The thermal detonator grenade will explode and damage the squad based on the countdown timer of the ability.

Article is somehow obsolete, some fixes/additions from me:

• Advantage now only guarantees 100% critical on next attack.
• Tactical Genius gives 100% TM after using special ability.

New debuffs were introduced:

• Shock prevents healing, TM gain, Buff gain.
• Daze prevents TM gain and counterattack.

Ships battles have some additional effects (but they sometimes reuse old icons):

• Damage immunity from Mace Window's Endurance.
• Master Plan from Admiral Acbar's Home One. Grants 100% TM and cooldown resets if special ability is used.
• Thrust Reversal of Millennium Falcon: +4% Evasion per stack, 1 stack lost on taking damage; at end of turn, 1 stack is consumed to gain 100% Turn Meter.
• Debuff Target Locked: debuff which does nothing by itself, but some abilities have additional effects if target is locked.

An up to date list of status effects can be found here:

https://docs.google.com/spreadsheets/d/1F86_j4IqMQ45HK4a0_3RNYb9mF4Jb0hTmbmMU2RoLEQ/edit#gid=0

The effects missing from the above post:

Buffs:

• Backup Plan - Unit recovers 10% Health at start of turn, Revives with 80% Maximum Health and 30% Turn Meter. Status Effect cannot be Dispelled.

• Bonus Protection - Unit is granted X Bonus Protection when applied.

• Bounty Hunter's Resolve - Status Effect cannot be Dispelled. Unit ignores Taunt. When defeated, revive unit at X% Health (Ability level dependent).

• Call to Action - Luke ignores Taunts during his turn and has +50% Accuracy, +50% Critical Chance, and +50% Critical Damage. Can't be Dispelled or prevented.

• Continuous Healing (HoT) - At the start of the units next turn, heal unit for X% amount.

• Critical Damage Up - Add 50% Damage to Critical Attacks made by this unit.

• Critical Hit Immunity - This unit cannot be Critically Hit, and will suffer normal damage from attacks.

• Deadly - Lowered Max HP, Massively Increased Damage. Status Effect cannot be dispelled. NPC Event Status Only.

• Frenzy - Give this unit 100% Turn Meter when any Ally uses a Special Ability.

• Master's Training - +25% (doubled on Jedi) Accuracy, Defense, Offense, Potency, and Tenacity. Can't be Dispelled or Prevented.

• Protection Up - Unit gains X% Protection.

• Protection Recharge - At the start of unit's turn, gain X Protection.

• Tenacity Up - Unit gains maximum Tenacity for duration of Status Effect.

• Secret Intel - +25% Potency and when another ally uses a Special ability they gain Secret Intel for 3 turns, then the cooldown of Illuminated Destiny is reduced by 1 for each ally with Secret Intel.

• Valor - At end of unit's turn, Endurance gains 20% Turn Meter, and reduces the cooldown of Convergence by 1.

Debuffs:

• Armor Shred - -50% Defense per stack. Raid Boss: -25% Defense per stack

• Burning - Deals damage over time and reduces Evasion chance to base 2%

• Critical Chance Down - reduces chance of scoring a critical hit

• Deathmark - While active, only units with Deathmark are targetable, and afflicated unit suffers damage equal to 50% of their max health when damaged. - Units with Taunt are still prioritized over units with Deathmark.

• Evasion Down - Reduces chance to evade an enemies attack

• Fracture - Unit Speed set to 0, cannot gain Positive Effects, bonus Turn Meter, or bonus Attacks. Raid Boss: -50% Speed (does not stack with Speed Down)

• Health Down - Max and Current Health total reduced by 20%.

• Plague: Unit loses 5% Health when inflicted and at the start of their turn, is immune to allied Dispels, and loses all stacks of Plague when healed to full Health

• Target Lock (Character Battle) - While Imperial Probe Droid is active, enemy units with Target Lock cannot gain buffs and have -25% evasion

• Tenacity Down - Unit loses all Tenacity for duration of Status Effect. - Note: Unit still has base 15% chance to resist detrimental effect.