Just like Etrian Odyssey III: The Drowned City, guild cards contain information like the number of steps your characters walked and the kinds of monsters you defeated. You can exchange guild cards through Streetpass or through QR codes, which you can upload to the Internet.
Atlus also confirmed, what accessory maker Hori leaked earlier. Special QR codes will unlock in-game content like quests and items. Atlus will update the official Etrian Odyssey IV site with these QR codes in the future.
Etrian Odyssey IV comes out on July 5 in Japan.