STAB means Same-Type Attack Bonus. A Pokémon normally receives a damage bonus when it uses a move that matches one of its own types. For example, a Water-type Pokémon using Surf benefits from STAB, while the same Pokémon using Ice Beam normally does not. The bonus is commonly 1.5 times the normal damage, although abilities, battle mechanics, and special forms can alter the result in some games. STAB is one reason a moveset usually includes at least one reliable attack matching the Pokémon’s type. It should still be balanced with coverage moves that hit common counters.
