eres is the tu/you form, so if i was talking about you i would say eres estupido because you are stupid. sois is the vosotros form, which is mainly used in spain, which means more of a "you all" is kinda thing.
so, eres, is just talking to someone as if you were saying "you"
if you were talkin to your buddies in spain you would use sois.
hope that helped