what does ! mean on a calculator?

! is a common notation to mean "Factorial".

The factorial of x is equivalent to [x] * [x-1] * [x-2] * [x-3] ... [x-n]

where [x-n] = 1

 

So say I want the factorial of 10.

This is 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1

And if I wanted 73!:

73! = 73 * 72 * 71 * 70 * 69 * 68 ... * 3 * 2 * 1