What is Open Source?
It's complicated and depends on who you ask...
Open Source usually means the 'source code' is openly available for modification and redistribution.. LIke most software, there are licenses that articulate the rights of "authors" and "users". However, unlike most software you buy online or at a store, this license usually makes sure the code stays "open" or "free". This usually makes the software free of charge. There are many variants of this approach that may or may not ask for compensation.
On a larger note, Open Source is a philosophy that encourages the open sharing of ideas and works in an effort torwards various forms of freedom. Open Source is a way to bring computers and technology to the poor in America and and the common person in third-world nations. It's a way to get modern software into our financially strapped schools. I emphasize the use open source software in my personal and professional life. I believe I get more value from it than the highly priced mainstream options. I do what I can to support the authors of the software I use.
Now, my explanation generalizes many points. Some might say it is totally off base. Judge for yourself. I've provided some places to start below:
"In production and development, open source is a philosophy, or pragmatic methodology that promotes free redistribution and access to an end product's design and implementation details" Wikipedia definition
A Wikepedia page on the 'open source movement'
An article from the most influential 'free software' advocate on the Earth: Richard Stallman's "Why Open Source misses the point of Free Software"