A colloquial phrase referring to the basics of a job, task, or concept. Similar in use to "the nuts and bolts" or "the ins and outs". Interestingly, all three of these phrases contain double entendres, but are intended for use in a generally non-sexual manner.
Oh, you're new here? Let me show you the balls and the butts of running the cash register.
Jesse, shut up. You really don't understand the balls and the butts of comedy.