Updated October 5, 2019: Updated October 5, 2019: Marvel has announced that the card packs contain seven cards each, and will be sold in 25-pack bundles. When assembled properly, the cards will reveal a “secret message in Krakoan.“
This announcement came during the publisher's Dawn Of X panel at New York Comic Con 2019.
(editor note: this story has been updated to correct the number of cards available from 25 to 7).
Updated September 13, 2019: Marvel is offering retails special Cards of X bundles tying into the promotion of the upcoming X-Men #1. The publisher is selling 25-pack bundles for $10 each, but giving away some bundles to retailers who opt in for the X-Men #1 launch parties.
Marvel doesn't specify what these cards are, but most commonly Marvel prints postcards listing various titles in an over-arching event - which X-Men #1 would be a part of with "Dawn of X."
Original story from September 6, 2019: Marvel Comics has unveiled plans for X-Men #1 midnight launch parties at participating comic book stores on Tuesday, October 15. In addition to the release of Jonathan Hickman and Leinil Yu's debut issue, the publisher plans "many exclusive offerings" including limited edition variant covers and unspecified other promotional items.
As part of this, Marvel has released a 6-part connecting cover by Mark Bagley that will be split up over the six "Dawn of X" Wave 1 launch titles. Take a look:
Check with your local comics retailer to see if they plan on hosting a party for this event.