A "plateau" is a kind of mountain with a flat top.

But there's another meaning of "plateau" that comes from its shape. On a graph, if you show something growing quickly and then staying the same for a while, it looks like the shape of a plateau.

So, to "hit a plateau" means to stop growing or improving at something. You usually use this phrase to talk about things you're learning:

A: How are your English studies coming along?

B: Uh, I've kinda hit a plateau.

You can also use "plateau" as a verb:

I was making gains for a while, but I've started to plateau.