It will do syntax colouring - and then also you can run the program and step through the code and see what happens to all the variables.
Just to be clear about things - most programs don't come with source code publicly available - they get compiled into machine code - and may be encrypted also to protect from hackers. The machine code is hard to read anyway!
Needs to be an open source project or one you have access to the code for.
And - if it is a complex project chances are won't be easy to understand what it does. Easiest programs to understand are probably the ones you get in programming books for beginners most of them come with example programs to try out.
It's different if you are working with javascript - the code embedded in webpages. I don't use that much so not able to give good advice. You can see the code just by using view source - but a lot of the code probably isn't in the page but included into it from elsewhere depending on the site, anyway others can probably answer about that better.