Basically it means that the neck was removed from the body and then put back in place. Specifically what it means could be any of a number of things so you should check it out. Did the neck just come unglued due to age or shrinkage or was it damaged or dropped? If it's visually obvious that it was "reset" - i.e. - added wood to the joint, screws or bolts (argh!), glue around the joint - then it may have been damaged and/or possibly badly repaired. Also you'll want to make sure it was reset at the correct angle but unless it's obviously way too high or too low you may need to have a luthier look at it which, in general, is a good idea if you're not experienced looking for problems with basses - kind of like taking a used car to a mechanic to check out before you buy.
On the other hand - if it's a good price, feels good to play and sounds good - and if it looks and feels structurally OK then go for it!
