Basically you spill your guts to this Frank guy (who has already received over 300,000 postcard secrets), and your secret may be posted or published. Obviously not everyone gets theirs posted, but just knowing you told Mr. Frank your SECRET is pretty intense.
Of all the secrets we all hold onto, it seems so crazy to be able to share them anonymously. I've been browsing through some of these posted on the site -- They are all really different. Some are funny, sad, angry, embarassing, creepy... I've never sent one, but I'm definitely tempted!