A fad refers to something that becomes very popular for a short period of time and then loses its popularity quickly. It can be a trend or a fashion that gains sudden popularity but is not expected to last long. The clue "It goes in and out" possibly refers to the temporary nature of fads, as they come and go quickly.
Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: It doesn't stay hot for long. Explanations are generated by AI and may be inaccurate.