how can I add an unofficial repository to ubuntu

I have to add http://download.opensuse.org/repositories/multimedia:/xiph/ repo to my ubuntu sources.list

It is written here:

There are package repositories that can be added to the distro package manager. You can think of it as a “PPA” or whatever your distro calls this.

But I don't understand, how exactly I can it's as PPA.

Solution

You are trying to add opensuse repositories to Ubuntu. They are different distributions. Find the Ubuntu PPA for the software that you want to install.