This was at the time early in the French Revolution when the 3rd estate had no power in the Estates General. The 3rd estate had to bear the burden of heavy taxes, while the clergy and nobility paid nothing. Yet, the 1st and 2nd estate held all the power. See, every estate got 1 vote in the Estates General. So it was basically inevitable that the 1st and 2nd would vote against the 3rd, and that was completely unfair. So Sieyes is saying that the 3rd estate literally IS France, and it deserves more power in the country where it makes up 90% or more of the population.
