See Margin Debt. You are borrowing from your broker for a portion of the investment. If Schwab is offering up to 50% margin and you have opened an account with them for $100,000, you can buy up to $200,000 worth of securities. The margin is the amount you have to put up; the broker lends you the rest. In this case, your $100,000 would represent 50% of the purchase price of $200,000 in stock.