To do battling, Pokémon use Moves (or "Attacks").
These Moves come in two types:
In battle, Fast Moves are the Moves performed when you just tap the screen. Every time a Pokémon does a Fast move it causes the opponent a little damage AND it generates some Energy, which, for your Pokémon, is indicated by a bar at the top of the screen under the Pokémon's name.
In battle, Charged Moves are performed when you tap-and-hold on the screen until a charge-up bar indicates it is ready, then letting go. Every time a Pokémon does a Charged move it causes the opponent a lot of damage BUT the Pokémon can only perform the Charged Move if it has generated enough Energy by performing Fast moves, which is indicated by the bar at the top of the screen being full.
NOTE: If you find you want to change one of the Moves for one of your Pokemons, you can make use of "TMs" that can be one of the rewards you get from defeating a Raid Boss. There are Fast TMs and Charged TMs so make sure you use the right one.