What is Polyprotic acid?

Example 10.15 (Phosphoric acid at $0.10\,\mathrm{mol}/\mathrm{L}$)

The predominant reaction is the first acidity. With $x = [\ce{H3O+}]$, $x^2/(0.10 - x) = 10^{-2.15}$; here $x$ is not small compared with $C$, so the quadratic $x^2 + K_{a1}x - K_{a1}C = 0$ is solved: $x = 0.0233\,\mathrm{mol}/\mathrm{L}$, $\mathrm{pH} = 1.63$. The second acidity ($\mathrm{p}K_{a2} = 7.21$) contributes nothing at this pH.