Stock prices change every day due to market forces. A high demand and low supply lead to an increase in price, whereas low demand and high supply lead to a decrease in price.
The supply and demand of a stock is influenced by the performance of the company. The stock price of a company that has reported increased earnings rises, as existing shareholders do not sell their stock, creating demand. Decreased earnings lead to shareholders offloading stock, creating additional supply that leads to fall in prices. Theoretically, investors' opinions and expectations of a company largely influence movement of its stock prices.