What it means
If losses push your account below the required maintenance margin, you get a margin call or your position is closed.
In practice
Margin is a good-faith deposit, not a fee: it is set aside while the trade is open and released when it closes. Initial margin opens the position and maintenance margin is the minimum needed to keep it.
Traders keep plenty of free margin so that a normal swing against them does not threaten the position. Margin requirements often rise overnight and around major events, which can affect trades held past the close.
What to watch for
The margin required says nothing about how much you can lose. A position can lose more than its margin in a gap, and on some accounts that leaves you owing money.