It’s like the deposit when you rent an apartment: the money is still yours, but you can’t spend it while the contract is running.
Example. You have $1,000 in your account. You open a $1,000 trade with leverage of 5. The margin is $1,000 ÷ 5 = $200. The other $800 is free, and you can use it for other trades; in the account line it's “Free”. When you close the trade, the margin comes back to your account together with the result.