For any webmaster (or webmistress) even semi-serious about their site, visitor statistics are a must. Not only for knowing how popular the site is, but also what part of the site is popular, where these people are coming from, and what they are searching for.
Impressions are counted as the number of pages viewed by users. Unique visitors are, as the name suggests, the number of people that actually visit your site. As an example, if Jack visited 4 pages and Jill 13 pages on your site, you will have gotten 17 pageviews and 2 unique visitors.

Pageviews give a good idea of how sticky your site is - the more impressions per user, the more users are looking deep into your site, reading what you have written.

Delving deep into visitor statistics gives you an idea of what to focus on, and what to fix.