It's called the "pattern day trading" rule. Wikipedia has a good article on it at Pattern day trader - Wikipedia, the free encyclopedia
Here are the key parts:
Pattern day trader is a term defined by the U.S. Securities and Exchange Commission to describe a stock market trader who executes 4 (or more) day trades in 5 business days in a margin account, provided the number of day trades are more than six percent of the customer's total trading activity for that same five-day period.
Under the rules of NYSE and Financial Industry Regulatory Authority, a trader who is deemed to be exhibiting a pattern of day trading will be subject to the "Pattern Day Trader" laws and restrictions, which is treated differently from a normal trader. In order to day trade: Day trading minimum equity: the account must maintain at least US$25,000 worth of equity.
The good side of this is that if you have the $25,000 equity (cash or stocks) in the account the day trading buying power is better. With $25,000 you can day trade up to $100,000 but you better be back under $50,000 by the end of the day and if you keep positions over night it subtracts from your day trading buying power the following day.
This applies to a margin account, but not to a cash account. However, in a cash account each trade ties up money for 3 days waiting for settlement.
Your broker can tell you exactly how all this works.