What does the song auld lang syne mean?

What does this song mean? My whole life, I don't know what this song means. I mean, 'Should old acquaintance be forgot'. Does that mean that we should forget old acquaintances? Or does it mean that if we happened to forget them, we should remember them, which is not possible because we already forgot 'em?
What does the song auld lang syne mean?
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