If you type in "mod" this will allow you to use the mod function.
So, for example mod(x,2) will show you the remainder of x after dividing by 2.
You can also use just numbers with the mod function. For example, mod(6,4) will show the remainder of 6 divided by 4 and output 2 in the expression list as the answer.
If you click the FUNCTIONS key on the Desmos keypad, you'll find more supported functions in the tabs.