The idea is similar to what Google is doing with the QR code window decals for Place Page business owners, but it's specific to Foursquare and requires that users download and install Foursquare's Android app.
Here's how it works: Users or venue owners can generate a printable 2D barcode -- using the venue address or Foursquare ID -- and stick it on the venue in question. Visitors can then use the Android application [download link] to snap a photo and scan the code. The scan automatically pulls up the venue in question in the Android Foursquare app for a faster checkin.
It's not the perfect solution to QR code checkins given that it only works for Android device owners, but it's a step in the right direction. We expect to see this type of logic and technology mature in the coming months, though adoption will require that business owners understand the purpose behind it, and that consumers embrace scanner apps like φ² Scanner.
You can watch a video of the application in action below.