The opening of the google adsense advertisements available on your site, and the display of another advertisement in the case of change in pages is termed as impression. when you open any page of your site, or you refresh any page, one impression is counted. if a visitor visits 5 pages of your website, then google counts it as 5 impressions.
Click through rate (CTR), means that what is the source of number of advertisements and number of clicks on your website. if your webiste has 50,000 page impressions, and in these impressions the total number of clicks are 1,200, then it means your PAGE CTR is 2.4%.
eCPM means effective cost per thousand impression, means, how much commission will you get, if your website has 1,000 impresssions. to calculate it, divide your earnings on your impressions, you will get eCPM.