% means Modulo operation.

https://en.wikipedia.org/wiki/Modulo_operation

if (x % 3 === 0) means x could be one of the values 0, 3, 6, 9, ..., x, 3x, ..., it could be -3, -6, -9 and ...

Sure,
"/" is indeed used to divide.
So: 4 / 2 is equal to 2
And:6 / 3 is also equal to 2

However when you use the % operation the result is equal to the remainder left after a division
So: 4 % 2 is equal to 0 because there is no remainder when you divide 4 by 2
However: 7 % 3 is equal to 1 because 1 is the remainder (3 X 2 )