A tuxedo is a type of formal suit worn for special occasions like weddings or black-tie events.
If someone says "He's going to wear a tux" they probably mean that he'll put on a fancy, all-black suit with a bow tie. You might hear this when talking about what to wear to a party or a formal gathering. The word "tux" is a shortened version of the longer name, but it's become so common in English that many people use it by itself. Some people even call a tuxedo just a tux, and the context makes it clear they mean the fancy suit, not something else entirely different.
(colloquial) Clipping of tuxedo
