The startup, founded by Stanford CS grad & former Google coding intern Joseph Huang, offered a way to let smartphones find their location indoors to an accuracy of less than 10 feet (2.5 m) using the ambient WiFi signals in the environment.
This isn't Apple's first foray with WiFi localization; the original, GPS-less iPhone used Skyhook's technology to provide more granular location info than could be gleaned from cellphone towers. But it is a big step towards interior location info for Apple's apps and OS, which is competitively important; Google Maps already includes interior floorplans for thousands of buildings, and allows crowdsourced contributions of public spaces.
The "SLAM" in WiFiSLAM refers to simultaneous location and mapping, a technique that autonomous robots use to build an environmental map while also keeping track of their location in space; in this case, rather than a robot, it's a phone doing the mapping. You can see Huang's own description of the tech in the (long) video below; the description of how SLAM works is at about the 30 minute mark.
Investors in WiFiSLAM included Google developer advocate Don Dodge (seen here in a TechCrunch interview) and Earthlink founder Sky Dayton. The company was offering an SDK for mobile developers to incorporate indoor location into their apps, but at this point the main site and the secondary footprint.io site are both offline.
It's safe to assume that the third party SDK is going to go away, and we'll see the WiFiSLAM technology appearing either in Apple's iOS apps or in the operating system itself. In one of Apple's previous high-profile mobile tech acquisitions, Siri took 18 months to go from independent product to part of iOS.