Okay kiddo, so you know how we use butter on things like toast or cooking, right? Well, sometimes we make more butter than we need, and that extra butter starts to pile up. It's like having a big hill made of butter, or a "butter mountain."
Now, imagine that butter mountain keeps growing and growing because people aren't using as much butter as they used to. It gets so big that people are worried there won't be enough room to keep it all! So, the farmers who make the butter and the government get together and come up with a plan to reduce the amount of butter being made.
They don't want to waste all that yummy butter, though, so they try to find other ways to use it, like giving it away to people in need or using it to make other products like cheese. Eventually, the butter mountain gets smaller and everyone is happy again!
So, to sum it up: Butter mountain is a big pile of butter that we don't need because people aren't using as much butter as they used to. We try to find ways to use it so it doesn't go to waste.