It's a very direct and blunt way to say someone smells unpleasant. It's important to note that this phrase is considered rude and impolite in most situations.
Here are some ways to say someone smells bad in a more polite way:
* "Tu as une odeur particulière." (You have a particular smell.)
* "As-tu remarqué que tu sens un peu fort aujourd'hui?" (Have you noticed that you smell a little strong today?)
* "Tu devrais peut-être prendre une douche." (Maybe you should take a shower.)
It's generally best to avoid saying anything at all, unless it's a close friend or family member who you know will not be offended.