A prime number has exactly two factors: 1 and itself. Nothing else divides into it evenly.

The smallest primes are 2, 3, 5, 7, 11, \dots. A number with more than two factors — like 12, which splits into 1, 2, 3, 4, 6 and 12 — is called composite instead.

Here is a picture of the idea. Take n dots and try to arrange them into a full rectangle. A composite number like 12 fits neatly into a rectangle (for example 3 \times 4) — those sides are its factors. A prime number like 7 stubbornly refuses: its dots only ever form a single straight line, 1 \times 7, because 1 and 7 are its only factors.

Press play, then replay — each time it tries a different small number.

One number is special: 1 is not prime. It has only a single factor — just 1 itself — and a prime needs exactly two. So the primes start at 2, which is also the only even prime (every other even number has 2 as an extra factor).

Khan Academy explains prime numbers here: