This refers to the remainder left over when a number is divided by another number.

Example 4 % 2 = 0, because when you divide 4 by 2, 2 divides perfectly into 4, leaving no remainder.

In the case of 5 % 2, the answer would be 1, since when you divide 5 by 2, the answer is 2, but with 1 left over - this left over part if the remainder and this equates to the modulus.

This is useful when determining whether a number is odd or even, as the modulus of an even number will always give ‘0’ when divided by 2, but the modulus of an odd number will always give 1 (this would work differently for values such as 0 and 1 obviously…).

In your example !== means ‘no equal to’, so maybe you can now figure out what the code means?