Wheel bearings ?

Ok i went to the shop today where I was getting my fork seals changed, new tires and a new chain put on.. and i had the mechanic show me what all was he was doing and he said that the wheel bearing was about to go sometime.. what is that and what are the dangers to leaving it too long (until i can get some more cash to have it BACK in the shop, and how much will this put me back?
thanks !