Start counting:
1, 2, 3, 4, ...
The counting numbers have no end. Whatever number we reach, we can always go one higher. They are infinite.
We call this set of numbers, the natural numbers. Why natural? It's because they're the numbers that arise naturally when counting things. And no, this doesn't mean that we should consider fractions and irrational numbers to be unnatural. It's taken years and several civil rights movements for them to gain acceptance. We wouldn't want to take that from them.