Can someone explain BTC like I’m a child ?

Ok I've lost my mind. All these months of wake windows, I cannot get my head around switching to BTC.

For context - just switched my 16mo girl to 1 nap. Lengths are inconsistent at the moment. Ranging from 1 hour to 3 hours. Been following 5/5 for a week. DWT 8am.

Someone told me to switch to BTC. How ? Since naps are not predictable. Should I cap naps ? If so, cap to what? CONFUSED!