When a Pokémon uses a move of the same type as one of his types, the STAB applies. This move is called "stabbed". The power of this move is increased by 25%.
It is interesting to take this mechanism into account when comparing two Pokémon's moveset, because a stabbed weak move can sometimes deal more damage than a non stabbed strong move.
Don't worry : the ratings shown on Goptimiz already take stab into account. More info on the formulas FAQ page.
Hitmonchan is a Fighting type Pokémon. Rock Smash is also a Fighting type move. Therefore, if a Hitmonchan uses Rock Smash, this move will be stabbed.
However, Pinsir is a Bug type Pokémon. If a Pisir uses Rock Smash, this move will not be stabbed and it will remain at 15 power.