A bankers' bank is kind of like a store where regular banks can buy things they need to run their business. Just like how your parents go to a grocery store to buy food for your family, banks go to a bankers' bank to buy things like money and other supplies they need.
But there's a special thing about bankers' banks - they only work with other banks, not with regular people like us. These banks help other banks by providing them with services that regular people can't get. For example, banks can use a bankers' bank to help them clear checks (meaning make sure the money gets from one account to another), or to get access to loans.
Think of it like this - let's say your parents own a grocery store. They need to buy bananas to sell in their store. They can either go to a normal store like Walmart to buy bananas, or they can go to a special place that only sells things to grocery store owners. That special place is like a bankers' bank - it only works with other businesses like your parents' grocery store.