=MOD(ROW(),2)=1
=mod(number,divisor) says take a number, divide it by another number and then leave me the remainder as a whole number. So for example, 5 divided by 2 is not 2.5 but is indeed 2 remainder 1. 6 divided by 2 is 3 remainder 0. 7 divided by 2 is 3 remainder 1 and so on.
The ROW() bit will simply use the Row number as the number for the MOD function. Whatever the Row number is it will be divided by 2 and the result will always be either 1 or 0. The conditional formatting simply plays on that to say OK, if the result is = 1 then colour the rows, hence every other row gets coloured. Even if you delete or add rows it doesn't matter. because the ROW() function will reflect whatever number the row happens to be.
Regards
Ken.............