A Pattern Day Trader is defined by the Securities and Exchange Commission as any stock market trader who executes 4 or more intraday trades within any 5 business day period in a margin account.
If anyone with a brokerage account value of $25,000 or less is flagged as a "Patter Day Trader", the person will have their margin powers revoked and will be limited to a cash account. This means that they will be unable to day trade with unsettled funds.
The SEC rationalizes this law by saying that day trading has "significantly higher risk" than long term holding, so they're trying to protect retail traders. I think this is unfair, because it is not the government's job to tell citizens how to spend their own hard earned cash. In fact, many state governments sponsor lotteries. Those are riskier.