A stock split is a decision by a company's board of directors to increase the number of shares outstanding by issuing more shares to current shareholders. For example, in a two-for-one stock split, an additional share is given for each share held by a shareholder. This means you would own twice as many shares, but each share would be valued at half the price of the original. It is what is referred to as a zero-sum-game, as you will be in the same position after the split than you were before it happened.