questions about '3-i' in 2048 game

hi , i am a beginner , when i am reading the tutorial of game 2048 , i met some difficulties.

please go to this website:

go to the row-155 , i am so confused .

what does “3 - i” “3 - j” mean? why we use the number ‘3’ ?
i have no idea how does it work.

You have a 2 dimensional fieldArray[rows][cols], with size 4 by 4.
To reference a specific field, you can thus use values from 0 to 3, e.g. top-left is [0, 0], bottom-right is [3, 3].
To iterate over the fieldArray, you either go from right to left (3 - j), or from left to right (j).