Imagine you and your friend Billy like to swap stickers. Billy has a lot of cool stickers, but he always seems to have more than you do, making it hard for you to get the ones you really want. You want to make sure that you can get a fair chance to get all kinds of cool stickers too.
Now let's say that your mom notices that Billy's mom is giving him a little bit of extra money each month to buy stickers, which is why he always has more than you. Your mom thinks this is unfair because it's like Billy is getting an extra advantage. So she decides to give you some extra money too, so you can buy stickers and have a fair chance at getting the ones you want.
That's basically what countervailing duties are. Sometimes, companies from one country import goods into another country and are able to sell them at really low prices. But it turns out that the reason they can sell them so cheaply is because the government in their own country is giving them extra help like subsidies or tax breaks. This gives them an unfair advantage over companies from the other country who can't sell their goods as cheaply.
So the government in the other country might decide to add extra fees or duties to the price of the imported goods to make it more expensive, which is like you getting extra money to buy stickers. This helps make sure that both countries' companies have a fair chance to compete and sell their goods.