it checks if the number $i (I am guessing $i value's is a number) is odd or even/

The modular or mod (%) returns the reminder of the division, such as if the statement is 4%3 it would be the reminder of the 4/3 division which in this case example is 1.

On the example code you gave if anything divided by 2 doesn't return 0 then the nominator is odd. I think it is a mathematical way to find odd or even numbers.