It can interact with a variety of serial port devices, including the X10 CM11 interface. It can be controlled or queried via command line calls, tcpip sockets, a web menu, or a gui interface. This article will give an overview of MisterHouse and give you pointers on how to download it and give it a test drive. By now I suspect the term 'open source' is pretty well understood, as so many of today's popular programs fall into this category. The idea is that instead of making just the binary of a program available, the source code for the program is included as well.
Authors do it for a variety of reasons.
It just feels good to be able to contribute back to the world wide community. I am fortunate enough to have a good day job, so money is not one of my goals. I have no guarentee that the author will keep enhancing it. People are more confortable with open source because not only is it free, but its future is not dependent on any one person. If money is a goal, I think a better HA business model might be not in selling the various programs, but in selling the service of installation and customization that is required to make it work in each home.