The Margin Level % is the ratio of Equity to Used Margin represented as a percentage. 

Margin Level % = Free Equity / Used Margin * 100 

This percentage is very important as it shows how much equity is available to cover the used margin. If the Margin Level dips below 120% the Terminal will light up red as a warning. If the Margin Level dips below 100% trades will be closed to free up margin.