A lockbox is a basically a padlock with a cavity inside where a key to your home can be placed. Only someone with an electronic key or the combination can get into the lockbox and access the key. 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. This would be inconvenient. Since almost every other house does have a lockbox available, if you do not allow one most agents will simply not show your property. 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 and cannot be removed. Only other agents can access the key that is located within the lockbox.
Next: Real Estate Commission