How would one explain the "main branch"?

I may need to explain a bit about source code management to non-programmers, and would like to hear other programmers talk about things like:

- the main branch
- a branch for a new feature
- merging the new feature into the main branch
- methodologies for adding new features that will be released before a major new version

I'd also like to hear about QA testing requirements as the you merge into the main branch.

I just have to explain this to non-programmers in a way which is simple to understand.  But I may learn something myself in the process.

thanks,
newbieweb