STAB (standing for Same-type attack bonus) gives a damage boost to attacks that are the same type as the Pokémon using the move. This boost increases the damage done by 50%. This boost can become 100% instead when a Pokémon has Adaptability as its natural Ability.
For example, a Fire-type Pokémon using a Fire-type move will gain a 50% boost because the attack is the same element as a Pokémon. STAB does not apply to moves that do a set amount of damage nor ones that do not use the normal damage formula, such as Dragon Rage and Seismic Toss.
Normal-type Pokémon also receive STAB, contradicting some of Nintendo's strategy guides.