A Stock Split is when a company decides to create and distribute more outstanding shares to the existing shareholders in order to make the stock price more affordable.
A company increasing its shares outstanding from 50,000 to 100,000 would be a 2 for 1 (2:1) stock split. As a result, existing share holders total shares held will double, and the price of each share would halve. If you have 10 shares prior to a 2:1 stock split, you would have 20 shares post stock split.
As a shareholder the overall value your total investment will not change when a stock split occurs. Instead, the individual stock price will decrease and your total shares will increase.