A "margin call" occurs when a brokerage firm requires an investor to deposit more money or securities into their margin account. This situation arises when the value of the investor's holdings falls below a certain level, known as the maintenance margin. The brokerage needs to ensure that there is enough collateral to cover potential losses. When a margin call is issued, the investor must act quickly to either add funds or sell some assets to restore the account to the required level. Failing to meet a margin call can result in the brokerage liquidating the investor's positions to recover the owed amount.