ppa

When I install a program on Ubuntu I'm sure I should add  ppa
what is ppa?

Suppose you have a software development
And is located in the Ubuntu repositories
You must upload the keys in reservoirs
So others can be downloaded
Another use for that can allow you to uploadand download  the source code in the repository is

example install package on ubuntu
 sudo apt-add-repository ppa:jon-severinsson/ffmpeg
apt-get update
apt-get install ffmpeg

how to work?
you want install ffmpeg on ubuntu but canot install because forget add ppa
stp1:
 sudo apt-add-repository ppa:jon-severinsson/ffmpeg
The first stage connect to Ubuntu repositories  and Launchpad
stp2:
When authentication finished (add key to ubuntu)
you Should type command
apt-get update
so
Ubuntu packages needed so that you uploaded

  Download  and added package to system

stp3: you type
apt-get install ffmpeg
install finshed

good luck