Unique Visitors - explained
A Unique Visitor is a separate/individual/distinct visitor.
If you have 100 pageloads on your site this could mean you had 100 Unique Visitors who each visited your site once.
OR you could have had one Unique Visitor who visited your site 100 times.
OR you could have 20 Unique Visitors who visited your site 5 times each.
Each Unique (separate/individual/distinct) Visitor to your site will be EITHER a first-time OR a returning visitor.
The sum of your first-time PLUS your returning visitors should always equal your TOTAL unique visitors for the day.