If someone wants to know and is willing to invest an hour to actually show interest in something you are enthusiastic about (not so many people usually), I ask them to watch Stallman explain it and help out, if there is a question of understanding.
Other than that, I tell people that in my free time I only do free software for ethical reasons. They can decide, ehether or not they are interested in what I do.
Make the software easy to install and easy to use. Make sure it's properly documented so users can consulted the help when they get stuck. That's what matters to "regular people".
What is the value of source code to "regular users" when they can't read the source code in the first place? (Publishing the source code doesn't have to be open source either.)
You could try and convince users that having the source code means it can be inspected or audited by programmers. But there is no guarantee that other programmers will inspect the code given the thousands of open source apps available. (How many programmers have the time to audit medium or large codebases voluntarily unless they are actual users of the app?)
We've seen examples of bugs in open source code lying dormant for years, challenging the widely-held belief that "many eyes make all bugs shallow".
The strongest arguments for publishing source code for "regular users" are:
- it's a guard or protection against malicious code or intrusive tracking. So that even if users can't read the source code of an app, it can still give them confidence the app doesn't do malicious things or spy on them. (Although that can't be guaranteed.)
- the app, if it's open source, won't lock users and their data into the app. And if the app changes direction or focus, other developers have the options to 'fork' the app. (But again that can't be guaranteed either.)