Margin is the amount of collateral to cover any credit risks arising during your trading operations.
Margin is expressed as the percentage of position size (e.g. 5% or 1%), and the only real reason for having funds in your trading account is to ensure sufficient margin. On a 1% margin, for instance, a position of $1,000,000 will require a deposit of $10,000.
The margin in your trading account needs to be equal or above 100% in order for you to be able to open new trades, unless the new trades will result in your trading account being fully hedged.