What does the rand between function do in Excel?

It allows you to generate a random number between a range. So say you want to get a random number between 1 and 100, then you would use the RANDBETWEEN function like this:

=RANDBETWEEN(1,100)

It is a dynamic function, meaning every time something changes on the spreadsheet, it regenerates new numbers. This will also happen if you press the F9 key. It is a good function to get a set of numbers quickly and to test functions with, removing the need for you to create a load of random numbers yourself.