NRML is a way you get exposure to more stocks than you the money you have in hand.
As an example, you have Rs. 1000/- in your account and you want to buy 100 shares of Reliance Communications, each share at Rs. 50/-
If you buy this in delivery, you need 100x50 = Rs 5000/- in your DMAT account. But with NRML, you can buy all the 100 shares worth 5000/- on debt, with 1000/- as a security deposit. But, you need to repay the debt within 7 trading days.
If you are aware of day trading,
NRML = Day Trading but you do not have to close the position on the same day - you can close anytime wihtin 7 days.