The "Arithmetic operators" help page (which you can get to via ?"%%") says

‘%%’ indicates ‘x mod y’

which is only helpful if you've done enough programming to know that this is referring to the modulo operation, i.e. integer-divide x by y and return the remainder. This is useful in many, many, many applications. For example (from @GavinSimpson in comments), %% is useful if you are running a loop and want to print some kind of progress indicator to the screen every nth iteration (e.g. use if (i %% 10 == 0) { #do something} to do something every 10t...