Open Source software is software that is low cost, usually free, and that is developed by online communities of developers and users working together, often as volunteers. The source code that makes the program work is always available for anyone to study. As with most software you agree to a license which says what you can do with, it but unlike propriatary software the license gives you freedoms and not restrictions. These freedoms include the freedom to study, copy, share and modify the software. This allows ideas and solutions to be freely shared and used to innovote and create solutions to new problems.

For a more detailed introduction try Open Source for absolute beginners from OSS Watch and  Schoolforge.org.uk has examples of usefull Open Source software to try.