She is the love interest of George McFly. Although she is married to George at the begginning of the movie, her son, Marty, goes back in time before they met.
However, he messes up the time line when Lorraine should have met George, making Lorraine fall in love with Marty.
Marty has to help George (who has a crush on Lorraine) to win her heart. They come up with a plan where he would pretend to take advantage of Lorraine at the Dance, and the wimpy George would "rescue" her from him.
However, the film's main antagonist Biff Tannen who also has a huge crush on her, comes in and sexually horrasses Lorraine instead of Marty.
George arrived and rescued her by standing up to Biff for the first time in his life. After initially being overpowered, he became angry when he saw a laughing Biff knock Lorraine to the ground when she tried to help him.
An enraged George bashed Biff in the face, knocking him out and Lorraine fell in love with the newly-confident George. This makes Lorraine fall in love with George and the two live on to get married.
This time more successful, because George now had more self-confidence after beating up Biff.