Complex sentence: When an anionic ligand replaces a coordinated water molecule the reaction is called a anation reaction, and when, in the reverse reaction, water replaces a ligand the reaction is called an aquation reaction.
Split into simpler sentences: When an anionic ligand replaces a coordinated water molecule the reaction is called a anation. The reverse reaction, water replacin a anionic ligand, is called an aquation reaction.