A lockbox is a basically a padlock with a cavity inside where a key to your house can be placed. Only someone with a key (electronic or mechanical) or the combination can get into the lockbox and access the key to your house. Having a lockbox available at your house makes it easy for other agents to get access to your house.
Without the lockbox, agents representing buyers would have to set appointments to meet you or your agent at the house so they could gain access and view the home. If you do not allow flexibility in showing your house, you will miss out on lots of potential buyers.
The listing contract specifies whether you allow a lockbox or not. It is locked into place, usually on the front door or walkway railing and cannot be removed. Only other agents can access the key that is located within the lockbox.