This is the amount of money that must be in your account if a broker (see) will lend you money to make a deal. These funds are "frozen" collateral, as soon as there is no free money left in your account, just the margin, the broker will send you a signal that you replenish your account. Otherwise, your transaction will be closed. To find out more about margin trading, read chapters 13 and 14 of the book “Secrets of stock trading” by Vladimir Tvardovsky and Sergei Parshikov.
Trading in financial markets involves substantial risks, including complete possible loss of investment capital. This activity is not suitable for all investors. High leverage increases the risk (Risk Disclosure).