What's that mean? "On margin"?
There are 2 different types of trading accounts, cash and margin. If you're on margin you can leverage yourself with your broker's money, but you pay interest on it (it's a loan) and there are rules you have to follow. If you've bought stock on margin the broker has the right to automatically sell and liquidate your a.ssets if your balance falls below the required amount.
Margin is also where the 3 trades per 5 day rule comes in. You can only make 3 day trades per 5 days on margin.
With cash it's all your money so there are less rules as far as requirements/maintenance goes. You can trade as much as you want on cash as long as it's with settled cash. For example if you start with $10,000 you can buy 10 stocks with $1,000 each and sell them all back in the same day. That would be 20 trades in one day (10 buys, 10 sells), or 10 day trades. On Margin you wouldn't be able to do this.
But the downside with cash accounts is that it takes 3 days for cash to settle. So using that same example above say you put in $10,000 all in one stock on a Monday and sell it the same day... from there it takes 3 days to settle. Now you have to wait until Thursday before you can re invest your money with settled cash and be able to sell.
You can buy in before Thursday, but it would be with unsettled funds and you would have to wait til Thursday to sell it. If you buy another $10k of stock on Tuesday then you're stuck holding it until Thursday no matter what.