Free margin
Free margin is the part of your equity that is not locked, and it is what can open new positions or absorb losses.
Free margin is your equity minus the margin currently locked by open positions, so it moves with every tick of an open trade.
When free margin runs out, no new position can open, and the broker refuses the order with a not enough money message.
Related terms
A definition, not financial advice. Trading carries a risk of loss.