Them's the rules.
It's an idiom used by particular groups of English speakers (Southern Americans, Brooklynites, for example) in colloquial speech. It means, those are the rules. If you see it in printed form, it will likely be in dialogue spoken by a person who uses a distinct dialect. Here are some examples of its use over the years. You'll see that each example contains a heavy dose of other dialectical pronunciations and informal ungrammatical constructions.
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