What is a second life sub

Assuming that you are referring to "sub" meaning submissive:

Submissives in Second Life are simply avatars that role-play or act like submissives in the BDSM lifestyle. Submissives in SL can be any race, sex, or form but are simply defined by their submissive actions.

If you are referring to sub as in food, or sub as in submarine: then, of course, those things are self-explanatory and they certainly exist in SL as well.