WiFiSLAM uses a combination of various methods to get better indoor locations. Obviously, WiFi and cell tower trilateration doesn’t work indoors. Instead, WiFi signals can be measured by any device to get an approximate location. In order for that location to be accurate, though, you have to use WiFi fingerprinting to get an idea of what the materials and construction of a particular building are going to do to WiFi signals. Enough scans in one place and you’ll have an accurate profile of a building that can be used to make a map.
The Next Web’s article goes into greater detail on how the company uses all the sensors in a phone together to not only create detailed maps but also track the routes people take through the building. Amazing stuff.