• At different pH, the structures of amino acids change because of ionisable nature of NH₂and COOH groups.

• what does it mean??

An amino acid has a basic amine group and an acidic carboxylic acid group. This structure internally transfers a hydrogen ion from COOH group to amine group leaving positive and negative charges on both. Such a structure is called a zwitterion.
At low pH: It means the medium is acidic i.e., there are enough H⁺ ions in medium so that the the carboxylate group tends to accept proton and form
 

At high pH: It means the medium is basic i.e., there is low concentration of H⁺ ions and high concentration of OH^- ions so that the NH₃⁺ of zwitter ion  loss a proton  to form NH₂. 

At neutral pH the amino acid tend to form Zwitter ion.