Used margin is the part of your account equity currently pledged as collateral against open positions — the sum of every position's margin requirement. It is not a fee and it is not spent: it is locked while the trades are open and released the moment they close.
The figure matters because of what sits next to it. Free margin is equity minus used margin, and margin level is equity divided by used margin — the ratio a broker watches for a margin call or a stop-out. Margin requirements are set per instrument and can be raised, so used margin can climb on positions you have not touched, quietly reducing the room the account has left.
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Guide
Margin Calls and Stop-Outs Explained
Balance, equity and margin level explained — how a margin call differs from a stop-out, which position closes first, and when a gap outruns the system.