A plateau is a high, flat area of land. The word has also been stretched to include a leveling off of progress––for a while the eighth graders having a sleepover were crazy, then their energy level reached a plateau.
You can see the word plate inside plateau. Think flat like a plate, or think about mountains that look like tables you could set with plates–-so flat the plates won't fall off. If you're a French speaker, this will be easier, as plateau derives from the French plat, "level."