means A plateau is an area of relatively flat high ground, or—figuratively—a stable period where progress levels off after a stretch of climbing.

from From French plateau, 'a flat thing, a tray or platter,' built from Old French plat, 'flat,' which traces back to Greek platys, 'broad, flat'—a distant cousin of words like 'place' and 'plate.' English borrowed the geographic sense in the early 19th century, and the figurative 'point where you stop improving' came later, when we noticed life has flat spots too.