Indoor maps basically helps you navigate large indoor places like airports, malls, large retail stores and more. It let's you find where you need to go and even handles the floor you are on. It is pretty cool.
Check out these demos:
If you own a building and want Google Maps to provide indoor directions, you can go to maps.google.com/help/maps/floorplans/ and go through the process.
Here is a demo of how it works in Ikea:
Here is Google's indoor FAQs.
Forum discussion at Google Maps Help.