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

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.

As Mathias711 said, these different effects cause a drop in that stat.*

• If it says ATTACK/DEFENSE BOOST, then it will increase that stat for your Pokémon.
• However, if it says DEFENSE/ATTACK DROP/BOOST CHANCE, then it has a chance to drop/boost that stat.*

* Stat boosts/drops are not permanent and only applies to that match.

** targets opponent's Pokémon if it says OPPONENT ATTACK/DEFENSE DROP/BOOST (CHANCE). Otherwise, it will affect your Pokémon.

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.