This is not my analogy, but a other poster's, on a different thread. I thought it was super simple to explain what the cap was.
So, imagine that every unit of energy (kWh) is an apple.
The government have said that the maximum price that can be charged for an apple is £1. The average household will eat 100 apples a year, at a cost of £100.
You don't eat as many apples (smaller households, for example), you only eat 50 apples a year. So your bill will be £50.
Fred down the road has a much bigger household, and eats 200 apples a year. His bill will be £200. This is more that the "capped average household bill of £100", because he has eaten more apples than average.
It doesn't take into account standing charges, but is a nice, visual analogy.
Again, not mine, I just borrowed it.