In real estate, you will hear about a margin in mortgage lending. It simply means that Adjustable-Rate Mortgages (ARM) offer a steady rate and, in time, the lender charges the borrower the interest rate as they use a different computing method. Since the rate adjusts over time, the bank adds up the margin based on a confirmed index rate. But the common case these days mean that the margin remains stationary throughout the duration of the loan, but the index rate changes over time.
For a deeper understanding, the index rate is the standard interest rate that shows the general market status. It also fluctuates depending on the status of the market and usually expected or preserved by a third party group. When certain changes occur, the index drive will definitely change your interest rate.
The margin is also the percentage points added in the index by the lender. It is usually established by the broker or the bank that functions as a lender when you try to obtain a loan, and usually does not change after closing. However, the amount established as the margin depends on the lender as well.
If you are looking forward to loaning, keep an eye on the margin as it differs from each lender in the market. However, you can try to negotiate your ideal margin sum and close the loan you are comfortable with.