In a margin trade (one using borrowed money to cause greater volatility in the trade), a margin call occurs when the trade goes against you (meaning the price goes down if you have a long position) so far that the value of the trade has gone down equal to the amount of margin you put up for collateral to open the trade.

For example, say you go long on ETH at $200 with 20x leverage (5% margin). You put down $100 as margin, buying you $2000 of ETH (or 10 ETH). A margin call would occur when the price of ETH drops by 5%, which would be $190 — meaning the trade lost $100, which is equal to your margin.

At that point your $100 margin would be completely lost, and your trade would be closed.

The price point that a margin call occurs is often referred to as the "liquidation price" as the exchange can liquidate the trade as soon as it hits that price.