Margin is the portion of your account a broker sets aside as a good-faith deposit to keep a leveraged position open. It is not a fee — it is locked collateral that is released when you close the trade. If losses push your equity below the required margin, the broker issues a margin call and may close positions automatically.
Opening 1 lot of EUR/USD at 1:100 leverage might require about $1,085 in margin. If your account is $1,200, only $115 of buffer remains before a margin call becomes a real risk.
This definition is for educational purposes only and is not financial advice. Trading involves risk.