STAB, short for Same Type Attack Bonus, is the 1.5 times damage bonus a Pokémon gets when it uses a move matching one of its own types. With the Ability Adaptability, that bonus rises to 2 times instead.
STAB has been part of the damage formula since Generation I, and it is one of the first things you check when building a moveset. A Garchomp using Earthquake gets STAB because Garchomp is Ground type, and the same Garchomp gets STAB on Dragon Claw because it is also Dragon type. A move that matches neither type gets no bonus, no matter how strong it looks on paper.
The bonus stacks with type effectiveness, which is where the big numbers come from. That Earthquake against an Electric type is both STAB and super effective, so the multipliers combine. Coverage moves exist to hit types your STAB moves cannot, but they start 50 percent behind, so a neutral STAB hit often beats a resisted coverage hit and sometimes rivals a super effective one.
A few mechanics change the calculation. Adaptability, held by lines such as Porygon-Z and Crawdaunt, raises STAB from 1.5 times to 2 times. Terastallization in Generation IX keeps STAB on your original types and adds it for your Tera type. Fixed damage moves such as Seismic Toss ignore STAB entirely. In practice, most serious teams are built around reliable STAB first and coverage second.
In play"My Garchomp runs Earthquake and Dragon Claw because both get STAB."