Sorry, I just copied that straight from my code and I should have explained it.
There are two arrays I will be writing and reading from, hence the array variable and the if statements. First is the "width" coordinate of the array and second is the "height" coordinate. So in C when you would usually do myArray{first}{second} (I can't use square brackets because this forum will think its a tag).
So basically all that's important is:
I want to write and read data to the array with the two indexes "first" and "second". "data" is the data I want to write to the array at the point {first}{second}.
Hope that cleared it up