Unique clicks
An estimate of how many separate people opened a link, as distinct from how many times it was opened.
It is an estimate, and it is worth knowing how weak one. Without a cookie there is no identity in a redirect, so the count is derived from what the request carried: the address, the user agent, the time. Two people behind the same office network can collapse into one, and one person on a train can become several as their address changes.
A shortener that sets no cookie at the redirect cannot follow the same person from one of your links to another either, so a visitor number is per link rather than per person.
Read it as a scale rather than a headcount. The trend between two weeks is meaningful; the absolute number is not.