I'm a PhD student. A paper for a conference got rejected. I haven't met my professor yet to discuss about it. But is it bad for my PhD performance? I mean, probably the professor will not trust my manuscripts anymore.
What field are you in? In mine (theoretical computer science), conference papers get rejected all the time: the better conferences tend to have acceptance rates of around 25-35%, and acceptance or rejection is a lot more random than you probably think (thanks to ff524 for the link). It's easy to judge what the best papers are, and accept those; it's easy to judge what the worst papers are, and reject those. For everything in the middle, it very much comes down to how enthusiastic the referees were and whether or not at least one member of the programme committee was willing to stand up and argue for your paper being accepted.
Assuming your paper wasn't one of the obvious-rejects, there's a decent chance that, if you resubmitted it unaltered to a different conference of about the same standard, it would get accepted. Hopefully, the referee reports made some constructive comments so you can improve the paper before resubmitting it, and have an even better chance. (Note that I wouldn't recommend resubmitting unaltered, unless you got no useful advice at all from the referees. As Paul Garrett points out in the comments, it's not unusual for at least one of the same referees to see the paper next time it's submitted and it's rather irksome to see one's advice ignored.)
In short, no. In long, this is a normal stage of going through a PhD: nobody gets all of their work published all of the time.
The best case scenario is that the reviewers have given you some useful feedback to work with, in which case now you're in a better position than you were before. Worst case, you got a flat reject with no helpful feedback (which happens sometimes: I've seen reviews that say "Insert review here"), but at least you'll still have something to talk over with your professor. In the latter case, try and work out why your paper was rejected: was the idea not considered novel enough? Was the explanation not clear enough? Or does this particular piece of work need some more data, more evaluation or more analysis before its publication worthy?
In either case, once you've talked this over with your professor, you should now have some idea of what direction you can focus your efforts on. This will help you get your paper ready for submission to a new venue.
EDIT: You mention in your comments that the reviewers said there was nothing theoretically new with your approach. This gives you a few options (which you should talk about with your professor). Firstly, if you disagree, then you need to make your case for this stronger (by citing similar papers in the field and showing how your approach differs). This is often easier than you might think; there are enough niches in research that you can often find a way to show that your work can be applied to specific cases in a new way, or in such a way as to tackle new problems. Secondly, reframe the work: the approach itself may not be novel, but you can frame it in a new way by, for example, comparing it to other approaches or by using it to form a position paper on a certain issue. Thirdly, if you've been scooped, or you feel this ground has already been trodden, then talk to your supervisor about exploring other approaches in this area.