A prime number is a number greater than 1 that has exactly two factors: 1 and itself. No other numbers divide it evenly.
Examples: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29. Notice 2 is the only even prime. Every other even number is divisible by 2, so it has at least three factors.
Non-examples: 1 is not prime (only one factor). 4 is not prime (factors: 1, 2, 4). 6 is not prime (factors: 1, 2, 3, 6).