A margin call is the term for when a broker requests an increase maintenance margin from a trader, in order to keep a leveraged trade open.
Margin trades require a certain amount of funds to remain open. If a trade loses money and the funds in your account are no longer enough to keep the position open, your provider will ask you to top up your account. This is a margin call.
If you top up funds, the position will remain open. If not, your provider may close the position and any losses incurred will be realised.