SLG = (1B + 2B×2 + 3B×3 + HR×4) / AB. Direct measure of power.
MLB average is around .400; .500+ is a strong slugger, .600+ is monster territory.
Big home run hitters lead SLG, but doubles hitters (like Suzuki) can approach .500 too.
SLG is often combined with OBP to form OPS (= OBP + SLG).