STAB means same-type attack bonus. If the attack is the same type as one of the types of the Pokémon using the attack, then the attack's base power is boosted 50%.
For example: A Blastoise using Surf increases the power of the move by 50%. Surf is boosted from base power 95 to base power 142, since both Blastoise and Surf are Water-type. However, a Blastoise using Ice Beam does not receive an increase in the base power of the move, since Blastoise is not an Ice-type Pokémon.
A dual-type Pokémon, such as Lapras, would receive the same-type attack bonus when using either Surf or Ice Beam, since Lapras is both Water-type and Ice-type.
Same-type attack bonus does not apply to moves that do a set amount of damage, such as Dragon Rage and Seismic Toss.
Hope this helps. :)