The word unique refers to a single open or a single click.
If a recipient opens a mail for the first time, that’s registered as a unique open. If they open that mail again, it’s not added to the unique open count in the dashboard.
Similarly, if a recipient clicks any link in a mail for the first time, that’s registered as a unique click. If they click on that link again, it’s not added to the unique click count. In case there are multiple links, each link will have its own unique click count.