I don’t know what there is to explain. .pacnew files are created wheen a piece of software requires a new configuration file, but the current one in use has been edited by the user. So it needs to be manually merged, so that it reflects what’s necessary.
But, and I don’t mean offense by this, if that idea goes above your head, and like you said concusses you now, it’s quite likely to happen again. And don’t feel bad, I didn’t get .pacnew files at first either, but if that’s not for you, please work through this essay: 
Edit:
About:
No reason to apologize for something when you’ve just started.