By posting about a new idea in public, you are releasing the idea to the public. This constitutes "prior art", which is one of the conditions under which a patent can be denied. Prior art means that someone has already done the same thing that you are trying to patent. If you post your idea, it is evidence of prior art, and thus no one else can patent the idea from that point on.
Note that *you* can still patent the idea for a short period after you make your public announcement. (Andrew mentioned a 1 year period, though I thought it was shorter than that. But I am not a patent lawyer.) Nevertheless, because it is your idea, prior art doesn't apply to you. (Since the prior art is yours to begin with.) But no one else can steal your idea and patent it out from under you once you've made the public post.
Hope this helps!
Adam