In order to balance the immense positive magic of the Clow Cards, Clow Reed created the Nothing Card, containing the negative magic equivalent in magnitude to the other fifty-two cards. True to the card's name, anything or anyone that comes into contact with it will cease to exist/return to nothingness.
The Nothing appears in the second movie and is terribly lonely in the beginning, unable to interact with others without destroying them. The only beings that can interact with her safely are the other Clow Cards. As a result, The Nothing tries to steal the Clow Cards (now Sakura Cards) so that she would not be alone again. Nothing continues to steal the Sakura Cards and destroys/unmake everyone and everything that Sakura holds dear; The Nothing likely views Sakura as a villain of sorts who stole her "friends" (the Clow Cards) and intends for Sakura to suffer the same pain of loss that she has felt for a long time.
In the end, the Sakura Cards return to Sakura, and Nothing falls into hysterical despair, believing that her friends have abandoned her. However, Sakura convinces Nothing that she can be her friend, too, together with all the Cards. She returns Nothing to a Clow Card form (we never see her card form except for in the beginning of the film) and then transforms it into a Sakura Card by merging it with the Nameless card to form The Hope. All the damage and deaths caused by Nothing are then miraculously undone by the rising sun.