It can mean a lot of things to different people, so beware of assuming you know what it means to the other guy. Exploration is key. Also, desiring to be dominant or submissive in sex usually has nothing to do with being dominant or submissive in a personal or professional life. In fact, the exact opposite is very often the case, the stereotypical examples of these being the ruthless businessman who wants to be dominated and the small guy who prefers to top. Assuming a sub has no will of his own is almost always a mistake, and assuming a big muscle guy wants to top everyone can also be a mistake.
Also, younger gays in this Age of Apps find it very easy to say "I want to be a sub for a daddy." What they actually mean can run the gamut from simple role-playing to classic BDSM. Again, assumption is a mistake and exploration is key.
As Shakespeare said, "There are more things in heaven and earth than are dreamt of in your philosophy, Horatio."