Unique page views are views by unique (different) visitors.  If one visitor looks at the same page 4 times, it would show up as one unique page view, and 4 total page views.  If another visitor does the same, then there would be 2 unique page views, and 8 total page views.

You'll see the "unique" reference in many different visitor statistics - especially total visits, and total unique visits.

This is not 100% accurate, because it is dependent on cookies stored in your individual browser to know whether or not a given user has visited the site before.  If a visitor clears their browser cache, and returns to the site again, it would register as a unique visit, since the record of their first visit has been deleted (the cookie, which is stored in the browser's cache).

From Wikipedia:

"When tracking the amount of traffic on a Web site, it refers to a person who visits a Web site more than once within a specified period of time. Softwarethat tracks and counts Web site traffic can distinguish between visitors who only visit the site once and unique visitors who return to the site.  Different from a site's hits or page views -- which are measured by the number of files that are requested from a site -- unique visitors are measured according to their unique IP addresses, which are like online fingerprints, and unique visitors are counted only once no matter how many times they visit the site. There are some ISPs that use Dynamic Host Configuration Protocol, such as AOL and cable modem providers, which use different IPs for every file requested, making one visitor look like many. In this case, a single IP address does not indicate a unique visitor."