A composite number is a positive integer that can be formed by multiplying together two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself. Every positive integer is composite, prime, or the unit 1, so the composite numbers are exactly the numbers that are not prime and not a unit.
It's not a math question. It's a question about a word. No different than if you asked, as in another question I read today, what legislation and autonomy are. Those are words about concepts in law or political science. But asking about their meanings isn't a question of law or political science.