Okay kiddo, have you ever gone to the store to buy something with your piggy bank money? And sometimes the price of the toy or candy you want to buy goes up or down? It's kinda like that with countries and their money.
The European Exchange Rate Mechanism, also known as ERM, is like a game the countries in Europe play to keep their money value steady. They agree on how much their money is worth compared to each other and promise to keep it that way.
So let's say we have two countries, France and Germany. They agree that one euro is worth 1.5 French francs and 2 German marks. But if the value of the euro goes down, then the French franc and German mark will be worth more compared to the euro. That means people in France and Germany might be more likely to buy things from their own countries instead of other European countries, because it's cheaper.
But the countries in the ERM don't want that, because they want people to be able to buy things from each other without worrying about the value of their money. So they promise to fix the exchange rate if it strays too much from what they agreed on.
They do this by buying or selling each other's currencies. Let's say the value of the French franc goes up too much. Then the French central bank will sell some of their francs and buy other countries' currencies, like German marks. That will make the French franc go down again, and the German mark go up. And that's how they keep the exchange rate steady.
So basically, the European Exchange Rate Mechanism is like a game the countries in Europe play to keep their money value steady by agreeing on how much their money is worth compared to each other, and if it strays too much from what they agreed on they promise to fix it by buying or selling each other's currencies.