You can borrow a maximum of 50% of the current value of the marginable securities in your account, either to receive cash that you might need unexpectedly, or for additional purchases of securities.
Or, as another example, if you have no securities and you want to buy $10,000 worth, you only have to put up $5000. You can borrow the remaining $5000 from the clearing agent at interest rates a couple of points above the "Brokers Call Rate". The interest charged for margin debit balances may vary from time to time.
A signed Margin Agreement must be on file before margin trading will be permitted. (This includes shortsales.)