what does ! mean in math

! is called the "factorial" symbol.

N! is the product on N and all smaller integers. (N must be an integer itself)

Here are some examples:
2! = 1 x 2 = 2
4! = 1 x 2 x 3 x 4 = 24
7! = 1 x 2 x 3 x 4 x 5 x 6 x 7 = 5040