A cardinal number tells you how many of something there are. When you count “one apple, two apples, three apples,” you're using cardinal numbers: 1, 2, 3, and so on. These numbers answer the question “how many?”

Cardinal numbers are different from ordinal numbers, which tell you the position or order of something. If you finish third in a race, that “third” is an ordinal number showing your position. But if there are twenty-five runners total, that “twenty-five” is a cardinal number showing how many.

Think of it this way: cardinal numbers are for counting (seven books, fifteen students, one hundred marbles), while ordinal numbers are for ranking (first place, second floor, tenth chapter). When your teacher asks “How many students are absent today?” she wants a cardinal number. When she asks “Who finished the assignment first?” she wants an ordinal number.

Cardinal numbers are essential in math: they're the basic building blocks of mathematics and the foundation of counting itself.