Margin level is percentage level of available free margin on the trading account. If margin level is lower then 100% then new orders will be impossible to open (except locking already opened orders), if margin level is lower then Margin Call level of trading account - all orders will be marked by Red color warning about close Stop Out level.
Margin level = (Equity / Used Margin) x 100
Margin level can be found in the Terminal window of MetaTrader