Maya means "that which is not." It refers to the Supreme Person's energy of illusion, which makes us think we are our bodies—products of the material world—even though we're eternal, spiritual persons, atmas. When we're under maya's spell, the attractive things in this world—wealth, fame, the opposite sex—appear real and desirable to us. We think we should be able to enjoy and control them, as we like. But really, everything here is under the control of time, and none of these temporary things can bring us lasting happiness.
Everything about Krishna, the Supreme Person, is completely spiritual. He always has been and always will be the supreme controller and enjoyer. But when we want to enjoy or control separately from Him—as if we were God ourselves—everything then appears to us as material, non-spiritual, separate from God, and exploitable. What we see then is maya, illusion, because in reality nothing is separate from the Absolute Truth, the Supreme Person, Krishna.