With more than 200 parameters to "tweak", the ardupilot-mega system can be a bit intimidating. Documentation is critical, but in a fast-moving project like APM, it is almost impossible for the documentation to keep up with the code. A major improvement in the code was the addition of inline documentation for the configuration parameters, several months ago. This documentation has been visible recently in the superb new configuration screen in Mission Planner, which provides a detailed description, units, defaults etc. for many key parameters.
Now, this documentation can also be found in the Wiki. Best of all, this wiki page is dynamically generated with every single new revision of the code, just like in Mission Planner. As new parameters are added, or existing ones modified, the developers can change a few lines of comments and the wiki page will update to reflect that change, instantly, from code to wiki.
Visit the new dynamic wiki page Parameters and their meaning