If something slots into something else, it means that it fits very neatly - just like a coin slipping into a slot in a machine. This sentence means that 'it' (presumably some kind of habit or practice) fitted very neatly into an old tradition. Or in other words, it followed on naturally from a long-established practice within the social group in question.

You need to provide more context and the complete sentence. We don't know what "it" refers to.