means A raised platform where performances happen, or, more broadly, a distinct phase or step in a process.

from From Old French 'estage' — a dwelling, a floor of a building, a position — which traces back to Vulgar Latin 'staticum,' rooted in Latin 'stare,' 'to stand.' So at heart a stage is simply a place where something stands; the theatrical raised platform came first in English, and the figurative 'stage of a journey' or 'stage of life' grew from the same idea of a fixed point you arrive at along the way. The 'stagecoach' kept the older sense too: a coach that ran in standing-points, stage by stage.