See margin debt. You're borrowing from your broker to invest money. The margin refers to the amount of money you have to pony up so that the broker pays up the rest.
If Schwab is offering up to 50% margin and you opened an account with them for $100,000, you can buy up to $200,000 worth of securities. Your $100,000 would represent 50% of the purchase price of $200,000 in stock.