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.