This usually means that, generally, it's not good, but not terrible. They invite people that they want, and those people will probably by admitted. But, they keep an additional list of people they'd want. So, come April 15th if they didn't get as many people as they wanted, they'd invite you, even if you didn't go to the interview weekend.
So, basically, you're not on the top of the list, but you still have a chance (probably not a GREAT chance, but still a chance).
That's what I've been told by professors.
