This is the free amount in your account. Basically, it is the funds with which you can easily open a new transaction or withdraw this amount from the account. Simply put, the useable margin is available money, whereas the used margin (see) - this is the money that the broker "freezes" when you open any transaction. They serve as a deposit until the transaction is closed. If you put 100 dollars into your account, your available margin will increase by this amount. If you start making transactions the useable margin will grow, if you save money it will decrease. To learn more about used margins, you can read chapters 13 and 14 off the book “Secrets of stock trading” by Vladimir Tvardovskiy and Sergei Parshikov.