Difference between Translucency and SSS

Whats the difference between translucency and SSS ?Is it Translucency is for plane surfaces like leaves,paper and SSS is for human skins,candles etc where light enters the object

Not exactly. Translucency is sort of a “fake” effect simulating light going through surfaces. In real life, most organic materials (and some non-organic too) have subsurface scattering effect: light doesn’t just go straight through, it penetrates the surface, is refracted, reflected, then refracted again, and finally leaves the surface, maybe at the other side, maybe at the same. Computing SSS for everything in your scene is way too expensive, and often (in case of, e.g. leaves when you’re looking at the whole tree) the effect will be negligible, so SSS is substituted by translucency.