Margin is the slice of your account balance the broker locks as collateral while you hold a leveraged position. At 1:100 leverage, opening a 1-lot EUR/USD position (~$108,500) requires about $1,085 of margin. If the trade moves against you enough that the remaining equity falls below the margin requirement, the broker issues a margin call or force-closes the position (stop-out).
Your "used margin" is the total locked across all open positions. "Free margin" is the portion of your equity still available to open new trades. "Margin level" is equity divided by used margin, expressed as a percent — when it drops to the broker's stop-out level (commonly 50% or 20%), the broker starts closing positions.