The MOD function returns the remainder after division. With a divisor of 1, MOD will return zero for any whole number.
We use this fact to construct a simple formula that tests the result of MOD. When the result is zero (i.e. when the number is an integer) the formula returns TRUE, triggering the conditional formatting.
When the result is not zero (i.e. the number has a decimal component, and dividing by 1 leaves remainder) the formula returns FALSE.