"Beep beep, Richie" was Richie's catchphrase in the novel, and was an inside joke among the seven kids. It wasn't meant to be something Pennywise created; it was Richie's, but Pennywise used it to reference him in a threatening way.
This catchphrase, in a sense, is taken from the Roadrunner, an old TV show character who would always say "beep beep". It could also be heard as just a car honking, but both possible references lead to the same conclusion--that in a way, it's meant to say "slow down". Since roadrunners continuously run, and a car's honking could signify "slowing down". Richie often takes his jokes really far, and his friends jokingly say that in a way of meaning "slow down". This was never really confirmed by Stephen King, but most of his audience interpret it this way.
But, it's just a catchphrase basically, if you don't want to analyze it too deeply. Beverly and Richie, who in the novel are actually very close and are sometimes mentioned in other Stephen King books, would always say that, and Beverly would often joke it to him. Since in the books, relationship across all seven (I know the film mainly focused on only Bev and Bill, but in the book it was shown for all seven of them to be super close), any sort of inside joke (i.e. Eddie's aspirator, jokes of Stanley being Jewish, Bill's constant repetition of the phrase "he thrusts his fist..." that sort of thing) are all significant because they're all special to some or all relationships in their gang.