STAB

STAB stands for Same-Type Attack Bonus.

This is a bonus that if the type of the attack matches the type of the Pokémon there is a 25% attack bonus.

For example, Twister is an attack that both Pidgey and Dratini can have. The base power for the attack is 25. While Pidgey will keep that 25 Dratini will have it at 31.25 (not showed in game).

In previous Pokémon games this bonus was of 50%.