Plesk is admin panel, something similar to DirectAdmin. Lets you (and your users) to administrate server via www, in browser.
Repository is something like a database of various linux programs (in this case only, repository as a term is much broader). Your software, responsible for managing (downloading and installing) another soft uses repositories to download from. Said software can be apt, or Synaptic package manager, or whatever else.
So in plain English, its like this: add atomic repository address to your repositories list so that you would be able to easily download and install Plesk control panel :)