My brother asked me this too when he first saw it, which I thought was funny, since he's read a lot of hard sci-fi--but I guess he didn't realize it!
Anyway, that gatekeeper of all knowledge--legitimate or otherwise--Wikipedia, has an article on hard sci-fi
, and starts off summarizing the meaning as follows:
Basically, in A*, I'm trying to stick to technology our current scientific knowledge thinks should be possible, either now or in the relatively near future, and not just theoretical (ie wormholes, sentient aliens) or plain fantastical (tractor beams, warp drives). This means ships in A* will not move faster than light (and in fact probably don't get much over 50% light speed, tops, since that's more or less the limit of what scientists think we can achieve with foreseeable means of propulsion), and there won't be talking aliens, hand-held ray-guns, etc.
There are various reasons why I'm restricting it in that way, but mostly I think it will help me build a more gripping, more believable story; you guys will know that I can't just invent some sort of deus ex machina technology to get myself out of a tricky plot situation--I gotta do it with tools everyone agrees upon.
So please do point it out if you think I'm cheating and inserting something that isn't sufficiently hard!
(Also it's just nice to insert "hard" into things. :P >_>)