This is a place where you leave letters or objects for another person.
I first read about them in a book called 'The Dead Letter Box' by Jan Mark, when I was a kid. The idea originates, presumably, from when there was no postal service, although in some parts of this country you'd be forgiven for still believing that...
In the book the kid leaves her address in a library book (I can see some of you getting paranoid over this but bear in mind this was written in 1982 and people were far less worried about their kids getting abducted) and meets her best friend through writing to her. Aaaah, ain't it sweet?
Anyway, try it out with your friends, it's kind of cool to stick a note in a tree and find a reply there a week later. Maybe I should get out more.....