In finance, a nominee is a person or company entrusted with the custody of securities or property of investors; all of your investments are held in his name while you retain control.
The securities are held in trust and the nominee is the legal owner, but you hold the real title as the beneficiary.
The broker can buy and sell on your behalf, but your funds will be protected if the brokerage goes out of business or your broker tries to defraud you.
The nominee company must be a neutral third party independent of the brokerage itself.