From the Overview: "A ballot drop box provides a secure and convenient means for voters to return their mail ballot. A drop box is a secure, locked structure operated by election officials where voters may deliver their ballots from the time they receive them in the mail up to the time polls close on Election Day. Ballot drop boxes can be staffed or unstaffed, temporary or permanent. Unstaffed drop boxes are typically available 24 hours a day and permanently anchored in place. Staffed drop boxes are typically available during regular business hours and monitored by trained workers during those hours. Whether you are standing up a temporary vote-by-mail program statewide or locally, or you plan to develop a permanent program of voting primarily by mail, there are a few things to know and consider when planning for the use of ballot drop boxes."
U.S. Election Assistance Commission: https://www.eac.gov/