STAB

STAB is an acronym for Same-Type Attack Bonus.

Details
This is a bonus that is applied to the power of an Attack, if the Type of the Attack matches the Type of the Pokémon. If the attack grants a STAB, a 20% Attack bonus will be applied.

Example
Twister is an Attack that both Pidgey and Dratini can have. The base power for the Attack is 25. While Pidgey's damage will remain at 25, Dratini's damage will be boosted to 31.25 (+6.25). STAB damage does not get displayed in-game.

Trivia

 * In the main Pokémon handheld series, the applied bonus is 50%.
 * Prior to the Gym overhaul and introduction of Raid Battles, STAB granted a 25% bonus.