The 1st column is the lines unique to the 1st file, the 2nd column is the lines unique to the 2nd file, and the 3rd one is the lines that are not unique. -1, -2, -3 allow to disable those columns. Pretty easy to remember.

Ah, HN downvoting neutral comments again and I don't like it, but I have a policy of not upvoting anything out of pity.