The affair allegations that derailed Kevin McCarthy's quest for the speakership, explained

A lot of people I speak to who aren't deeply involved in American politics are a little confused about what, exactly, is up with the House Republicans these days. Why did Kevin McCarthy remove himself from the running for speaker without even having a vote? And if the problem with McCarthy is that he wasn't conservative enough, why are people talking about Paul Ryan — who isn't any more conservative than McCarthy — as possibly being able to step into the breach? What, in other words, is actually going on?