The “dead drop”, also known as “dead letter box”, is a special location that is used to get information (or objects) from one spy to another spy, without the need for the spies to meet.
Anything can be a dead drop. For example, a cavity in a tree, or under a bench in a park. But anywhere where you can easily hide and retrieve messages, without drawing attention to yourself, will work.
On another location, known as the “signpost” a spy would leave some mark to signal the fact that a dead drop was made. When this sign (which can be just about anything both spies have agreed upon) is spotted, the 2nd spy would then go to the dead drop to collect.