I'm a new user. I don't understand why I can't tell users what is wrong with their work. You can only do that when you have reached reputation=50? I also don't understand anymore what is meant by answer? I'm suppose to provide a full answer since I don't have commenting privileges? I need your definition of answer to be defined? If it is defined somewhere, I don't know where. I'm still trying to work out the navigation of this site.
Yes, this is the idea. As long as you have not reached 50 points, you can only do posts (questions and answers). An answer should be an actual answer to the questions; it can be a partial answer too, but it should be substantive.
The reason why you cannot tell users what is wrong with their work is that, as unfriendly as this may sound, "we" first want some proof that a new user has some clue what they are talking about.
Moreover, this limit is also a form of spam and noise protection.
I think the existing answer probably makes more sense to someone who's already familiar with how the site works, so let me try to explain the policy from a more abstract perspective:
This policy is meant to deter people who know absolutely nothing about math. If you belong here (which you obviously do), you'll accumulate the required reputation threshold nearly immediately. For instance, it took me one day to hit the threshold, and I wasn't trying; if I had been, it probably would have taken about two hours from the time I created my account.
If you allow commenting for anyone, the following happens: people will Google a problem off of their homework assignment, end up here at a question that's not really related, then start asking incoherent questions in the comments to the answers.
As such, the site requires users to prove that they're serious by accumulating a few reputation points before commenting. This is annoying when you know what you're doing -- it's always a bit cringeworthy when you see a comment on MathOverflow from some legendary mathematician saying something like, "Apparently I don't have enough reputation to comment, so I've put this comment into an answer..." But it's quite easy to accumulate enough points to comment quickly: just answer a couple of basic questions and you're set.