What is this weird sword Icon that appears when I use a tool or weapon?

In minecraft 1.9 you cannot spam weapons. They now have cooldown timers. Weapons make most damage if they are fully charged. So you have to wait some time after you hit.

EDIT:

Note:

• In Minecraft 1.9 an axe makes more damage as a sword of same type. It has longer cooldown tho.
• The fastest cooldown has a hoe.

Here's what the Minecraft Wiki says about it:

Combat

• New “attack strength” combat mechanic:
  • A meter shows up after switching items or attacking.
    • Damage done depends on the fullness of the meter, with a quadratic reduction (attack does 20%–100% of normal damage as the meter fills).
    • Fills at different rates depending on the new attack speed attribute (see table on the changes to gameplay section for a complete list).
      • The haste effect will cause the meter to fill faster
    • Can be displayed next to the hotbar (left or right based on the player's main hand), under the crosshair or turned off.
      • Looks like the icon for the strength effect on the side of the hotbar, or like a dagger under the crosshair.
      • This is controlled in video settings.
  • A cooldown animation is displayed of the tool slowly being lifted up, when first switching to it.
• Plays sounds when attacked strongly, weakly, and parried.

You can also check the Mojang update article.