An amphoteric substance is one that can act as an acid or as a base, depending on pH. Water (below) is amphoteric. Another example of an amphoteric molecule is the bicarbonate ion  that is the conjugate base of the carbonic acid molecule H2CO3 in the equilibrium
but also the conjugate acid of the carbonate ion  in (the reverse of) the equilibrium
Carbonic acid equilibria are important for acid–base homeostasis in the human body.
An amino acid is also amphoteric with the added complication that the neutral molecule is subject to an internal acid–base equilibrium in which the basic amino group attracts and binds the proton from the acidic carboxyl group, forming a zwitterion.
At pH less than about 5 both the carboxylate group and the amino group are protonated. As pH increases the acid dissociates according to
At high pH a second dissociation may take place.
Thus the amino acid molecule is amphoteric because it may either be protonated or deprotonated.