Same-type attack bonus, commonly abbreviated as STAB, is a 50% boost to the power of an attack when the attack is the same type as one of the types of the Pokémon using the attack. This results in a 50% increase in damage (100% if the Pokémon has the Adaptability Ability.)
Same-type attack bonus 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.
Source: Bulbapedia's STAB page
