A not-for-profit (also called nonprofit) organization is one organized so that no profits from the operation of the organization go to private individuals. Most SF clubs and SF cons are organized this way. In the US, non-profit corporations are chartered by the states. (501(c)3 status can be granted to a non-profit corporation by the IRS.)
Note that fannish conventions are not just Not-for-Profit, but don't pay the organizers or volunteers anything.