What does "attack drop" mean for a Charged Attack in Trainer Battles?

I assume that for Pokemon Go Battle League the "Trainer Battles" details for a Pokemon are what matters. So I was looking at the card for a Charizard and under the "Trainer Battles" for its Overheat attack it has an "ATTACK DROP" note (see image below). What does "ATTACK DROP" mean? When and why does it appear or apply?

Other notes I have seen:

• DEFENSE DROP
• OPPONENT DEFENSE DROP
• OPPONENT DEFENSE DROP CHANCE
• ATTACK BOOST

Solution 1:

In Trainer Battles, your attack and defense stats are scaled by a BuffMultiplier, whose value starts at 1. Certain charge moves may have a side effect of boosting or dropping those stats in stages. The range of values is [1/2, 4/7, 2/3, 4/5, 1, 5/4, 3/2, 7/4, 2].

The notes under certain charge moves have four parts - two required and two optional - to describe the move's side effect.

<opponent> [attack|defense] [boost|drop] <chance>

If chance appears, the move has a certain probability for that side effect to occur. The range of possible probabilities is [1/10, 1/8, 1/5, 3/10, 1/2]. Technically, all side effects have a probability, where the lack of chance means a probability of 1.

Solution 2:

This will change your (or your opponents) attack or defense stat in a trainer battle. In a raid battle, there are no (or not yet) such stat changes. The impact is roughly 25% extra attack/defense per stage (there are attacks that can increase it with two stages, such as Ancient Power) in case you increase it. The decrease (on yourself or opponent) is around 20%. Take a look around here for all moves and percentages.