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