FAQ:What is Open Source?
Open source software is software where the source code (the language programmers use to make computers do their jobs) is available to everyone. Anyone can see how the code works and can change it if they want to make it work differently. The opposite of open source is closed source, where the source is not available to everyone.
Open Source is better than Freeware, because freeware is closed source, and can be easily moved to commercial software, Open Source software will be free forever.