What is Levelling effect?

In water, every acid stronger than $\ce{H3O+}$ reacts totally to give $\ce{H3O+}$, and every base stronger than $\ce{OH-}$ totally to give $\ce{OH-}$: their strengths cannot be distinguished. This is the levelling effect of the solvent. Weak acids and bases in water have $0 < \mathrm{p}K_a < 14$, the couples of water itself ($\mathrm{p}K_a(\ce{H3O+}/\ce{H2O}) = 0$ and $\mathrm{p}K_a(\ce{H2O}/\ce{OH-}) = 14$) bounding the scale.