STAB

STAB is an acronym for Same-Type Attack Bonus.

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 a Dragon-type Attack that both Pidgey and Dratini can have. The base power for the Attack is 45. While Pidgey's damage will remain at 45, Dratini's damage will be boosted to 54 as it is Dragon type. STAB damage is not 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.