It’s simple alphanumeric code: 1=A, 2=B, 3=C and so on. 1312 is ACAB, it means “All Cops Are Bastards”, a motto that’s supposed to remind us cops aren’t on our side.
Which isn’t to say that every cop ever is a real dick of a guy (even if a lot of them are), but that it’s their job to be antagonistic toward us.
tbh I don’t know. I think at one point it served a purpose but now it’s just a cultural artifact.