What does it mean by stock split?

Stock split happens when a company divides its existing shares into several new shares to increase the liquidity of the shares. Although the number of shares outstanding increases, the total dollar value of the shares remains the same compared to the amount before the stock split, because the split does not add any real value.

As an illustration: Amazon (AMZN) stock is currently worth $2,000/share and Amazon announced a 1:4 stock split.

If you owned 5 AMZN shares before the stock split, then after the stock split you will own 20 AMZN shares with a value of $500/share.