A margin call is basically a warning from your broker signaling that you need to deposit more money into your trading account because your Margin Level has fallen below 100%.
When your Margin Level falls below 100%, it means your free margin is finished and any further losses will begin to deplete your initial margin which was locked up as collateral.
If you are not able to deposit more funds you better hope your trades become profitable because if they don't, your broker has a tolerance limit called the stop-out level so once your margin level falls to this stop-out level, all your trades will be forcibly closed starting from the least profitable ones.
Example if your broker has a 20% stop-out level, then once your margin level falls to 20%, all your trades will be closed automatically.
The weird thing about margin calls is that unlike the name, you do not actually get a phone call (except you are a VIP customer trading thousands of dollars). Instead, on MetaTrader platforms, the margin call comes in form a blinking red light at the bottom of your toolbox.
When trading, I always keep an eye on my margin level because the more trades you open, the more your margin level gets lower.
The ideal margin level is above 100%, so you must always ensure it stays above 100% by either depositing more funds, or closing unprofitable trades etc.