Explain · Like I'm five version
The robber in Catan in terms of a pickpocket
The robber in Catan is just a mean pickpocket who loves to steal your favourite toy.
When you play Catan, you might get a nice card that helps you build a big road. But then, the mean robber moves onto your land and takes a card away from your pile. It is just like a silly thief walking up in the street and reaching right into your pocket to grab your best marble while you are busy looking at the ducks.
The thief does not care if you were saving that card for a long time. He waits until you have lots of cards and then he swoops in to take one for himself. It is very cheeky, like someone sneaking a biscuit from your tin when you are not looking and then pretending they did not do it at all. He just wants to stop you from winning the game.
Sometimes the robber stands in the middle of a forest and stops everyone from getting wood for their houses. It is a bit like a grumpy person sitting on the slide at the park so no one else can go down. You have to roll a lucky number to make him move away to someone else, which is the only way to get your things back and finish your big tower.
Where the analogy breaks downThe robber does not actually keep the stolen card in his own pocket, he just tosses it back into the bank.