Okay, so imagine you really love candy. Every time you go to the store, you buy some candy. Sometimes there's a lot of candy, and sometimes there's not very much. When there's not very much candy, it's really expensive and hard to find. You might have to go to a different store or pay more money for the candy you want.
Now imagine the people who make the candy have a plan to make sure there's always enough candy for everyone who wants it. They create a big storage area, called a buffer stock. Whenever there's a lot of candy available, they add it to the buffer stock. When there's not very much candy available, they take some candy out of the buffer stock and sell it to people who want it.
This way, there's always a steady supply of candy available, and it doesn't get too expensive or hard to find. The buffer stock scheme is like having a secret stash of candy that you can use when there's a candy shortage.
The same idea can be used for other things besides candy, like food or oil. When there's plenty of something, it's stored in a buffer stock so that it can be used later when there's not enough. It's a way to make sure everyone has access to the things they need, even when there's a shortage.