Lady Macbeth is the wife of Macbeth, the eponymous protagonist of the play. She is one of Shakespeare's more powerful and interesting female characters, in part for the way she illuminates the fundamental ambiguities of gender roles in her period. To be a traditional "good" wife, she needs to help Macbeth succeed in his ambitions, but that involves taking actions that are not traditionally feminine, and thus she must "unwoman" herself to be a good wife and woman.
Lady Macbeth, on hearing of the witches' prophecies, helps strengthen Macbeth's resolve to kill Duncan, and plans both the murder and the cover up, although she herself cannot bring herself to kill the old man in his sleep because he reminds her of her father. Eventually, she is driven insane by guilt and commits suicide.
The main function of Lady Macbeth in the play is to strengthen Macbeth's resolve and give him advice.