Imagine that you have $10,000 on your account account, and you have a losing position with a margin evaluated at $1,000. If your position goes against you, and it goes to a $9,000 loss, the equity will be $1,000 (i.e $10,000 - $9,000), which equals the margin. Thus, the margin level will be 100%. Again, if the margin level reaches the rate of 100%, you can't take any new positions, unless the market suddenly turns around and your equity level turns out to be greater than the margin.
Whether you have assets in a securities account or in a futures account, your assets are protected by U.S. federal regulations governing how brokers must protect your property and funds. In the securities account, your assets are protected by SEC and SIPC rules. In the futures account, your assets are protected by CFTC rules requiring segregation of customer funds. You are also protected by our strong financial position and our conservative risk management philosophy. See our Strength & Security page.