STAB (Same-Type Attack Bonus) is a key mechanic to any Pokémon game. If a Pokémon uses an attack of the same type as its own, they will receive a 50% damage bonus.
Rattata, a Normal-type Pokémon, uses Tackle, a Normal-type attack. Tackle has 50 Base Power, so it would look like this:
50 Base Power + 50% STAB = 75 Total Power
Squirtle, a Water-type Pokémon, uses Water Gun, a Water-type attack. Water Gun has 40 Base Power, so it would look like this:
40 Base Power + 50% STAB = 60 Total Power