To install plugins you need to use an app to ssh into your server and edit a text file. The git repos are where the plugin code lives, and your part is adding those URLs in a list in the text file and then rebuilding Discourse so it retrieves the plugins and installs them for you.
You still need to log into to your server remotely (search ssh) and edit a text file from the command line (probably with vi or nano), and then run the commands in the post you referenced. Read through it a few times, it might be more understandable now. 