"!" means "factorial" and it's a fast way to indicate multiplication:
0! = 1 (a special case)
1! = 1 (a special case) also: 1! = 1 x 1!
2! = 2 x 1 = 2 also: 2! = 2 x 1!
3! = 3 x 2 x 1 = 6 also: 3! = 3 x 2!
4! = 4 x 3 x 2 x 1 = 24 also: 4! = 4 x 3!
5! = 5 x 4 x 3 x 2 x 1 = 120 also: 5! = 5 x 4!
etc.
Another way to define !: 0! = 1 and, if n is a whole number > 0: n! = n x (n - 1)!