Working with open source software

[Yamin]

Hi I was wondering if you could help me understand working with open source software. As I understand its possible to make your own alterations to it, add extra functions and what not. My questions is how do you do it-I know its a bit out of my depth but still I would like to know how its done. So if you could tell me or direct me to some explanatory material I would really appreciate it.

I tired some research online- but maybe I'm not using the right technical words.

Thanks in advance.