This is the process of dividing one expensive stock into several cheaper stocks of the same company so that the total value does not change.
Example: You have 5 shares of TSLA on 31.03 at a price of $1,000/share, that is, a value of $5,000. The company announces that from 01.04, a 10-for-1 stock split is being carried out, that is, 10 shares will be given for 1 share.
That means that on 1 April, at the opening of the market, you will already have 50 (5 * 10) TSLA shares at a price of $100 each. In this case, the total cost will remain the same at $5000.