A page impression is when sometimes loads a page from your site. If it loads a page 2 times, you get 2 impressions. If it loads 2 different pages, again, you get 2 impressions

It depends on how you are using the term. Adsense? SERPs?

If it is adsense than it is how many times the page or pages containing your AdSense Ad was shown to visitors (viewed but not necessarily clicked on)

If you meant SERPs or organic search results (which can be found in your webmaster tools dashboard) than it is how many times your site was displayed for a specific search query (again, but not necessarily clicked on)