Complex sentence: The hydroxide ion is a good benchmark because it is the strongest base that can exist in a water solution; stronger bases neutralize water as an acid by deprotonation, to produce hydroxide (and protonated superbase).
Split into simpler sentences: The hydroxide ion is a good benchmark because it is the strongest base that can exist in a water solution. Stronger bases are neutralized by water acting as an acid, to produce a corresponding hydroxide (and protonated superbase).