You could put
in row 1 and copy it down
(or the opposite
Mod simply returns the remainder. For example in row 3, it would return a 1 since 2 goes into 3 once with a remainder of 1. This is just used to check for even or odd rows. Excel interprets 0 as false, and a positve number as true.
A1 - enter a 1
A2 - leave blank
A3 - enter the formula & copy down..
row() / 2
If the row number / 2 leaves a remainder, then the row number is odd.
mod(row(),2) returns the remainder of the row() / 2 calculation.
It's use in the if() can be 'expressed' roughly as follows:
if the row number divided by 2 gives us anything, add 1 to A1, otherwise nothing.
(This works because excel will treat 0 as false and other numbers as true, which can be exploited within the true / false checking of the if statement).