Margin in its simplest form is a loan. As of now, you can trade with whatever amount you have in your Bitbns wallet. So, if you have 100 XRP, you can trade with 100 XRP only.
What if you have a strong understanding that the market might move in a particular direction, and you want to invest more than what you have? This is where margin comes into play. Here, a set of lenders can lend you a certain amount, and with that you can invest more than what you have. The lenders earn interest, the borrowers get to place larger orders. Overall a win-win.