This is a certain amount that the broker (see) temporarily "freeze" in your account as collateral for your opening of any transactions. With the remaining (free) money you can open new deals. However, if the market suddenly goes your way and you will be in the red by the sum of all available funds, the broker will notify you and ask you to update your account ("margin call" - see). If you do not, and your losses continue to grow, there will be stop-out (see), so the broker is forced to close your deal with a minus. Each tool has its own margin size. 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.