A Dead Drop is simply an agreed upon spot where you and your contact can leave a message for one another without actually meeting in person. They’re typically used in conjunction with a “Signpost,” or a place where a signal is left which conveys the drop has been loaded.
The Hollow Spy Bolt is what’s referred to as an Overt Dead Drop, which would be disguised as an everyday item that could be left in plain sight. The hope is that no one would give it a second thought if it was properly placed in its environment. With a simple counter-clockwise twist, the bolt head opens to reveal a hollow compartment.