What happens
Each broker sets a margin call level, often expressed as a margin level percentage (for example 100%). When your margin level falls to it, you are warned that you need to add funds or reduce positions.
If losses continue to the stop-out level, the platform starts closing your positions automatically, usually starting with the biggest loser, to stop the balance from going negative.
How to avoid it
Margin calls are a symptom of positions that are too large for the account. Using a stop loss on every trade and keeping position sizes small relative to your balance makes margin calls very unlikely.