I am an engineer but I don’t have any development background. My boss gave me a Mac Mini with Xcode in it, and looks like someone sent me some source code to be used with phonegap.
So my question is, if I install (?) phonegap, will I be able to use Xcode in a normal way in the future, or is it an irreversible process that changes Xcode completely?
No, it will not affect Xcode at all. Now, you will just be able to select the phonegap application type when creating a new Xcode project if that is what you want to use.