What it means
Many platforms skip the call and close your position automatically.
In practice
A margin call is the platform telling you that the account can no longer support its open positions. The choices are to add funds, reduce the position or let the broker close it.
Traders who size positions from a stop-loss rarely see one, because the stop takes them out long before margin runs short. Getting a margin call at all is a sign that position size, not the market, is the problem.
What to watch for
Adding money to meet a margin call usually means funding a losing trade that had no exit plan. The broker may also close positions at the worst moment without waiting for you to respond.