Here’s a few snippets from http://www.flatstanley.com/how.html that explains what Flat Stanley is:

“In 1995, Dale Hubert, a Grade 3 teacher in London, Ontario, Canada, began the Flat Stanley Project. He invited other teachers to take part by hosting flat visitors and to encourage their students to write their own Flat Stanley journals.

Students make paper Flat Stanleys and begin a journal with him for a few days. Then Flat Stanley and the journal are sent to another school where students there treat Flat Stanley as a guest and complete the journal. Flat Stanley and the journal are then returned to the original sender. Students can plot his travels on maps and share the contents of the journal. Often, a Flat Stanley returns with a pin or postcard from his visit. Some teachers prefer to use e-mail only. Especially creative hosts send the Flat Stanleys back with pictures, souvenirs, stories and reminders of the visit.

The Flat Stanley Project is a penpal activity- but it’s far more than that, too. By sending a Flat Stanley it’s as if the senders and the recipients have a mutual friend- a little flat guy they can both talk about.”