To lurch is to suddenly move — usually forward. If you are on a ship that lurches a lot during a storm, you may find your body lurching in one direction and your stomach going in the opposite one.
The verb lurch can refer to any abrupt movement, but it often has the sense of a sharp turn up, down, or sideways. If you invest in the stock market, you have to be prepared for sudden losses and gains as the stock market can lurch up or down on a whim. The verb also means to make sudden jerking movements when walking, as if you cannot control your movements.