Impression is the number of times your ad is displayed.
Click is the number of times your ad is been clicked

Clicks is where someone presses your link to your website and impressions is where your website shows in the search

A click through is the real amount of times someone has taken their curser, placed it on your marketing image and used their mouse to click on that image and be taken to your website. An ad impression is basically the amount of times your ad, whether it’s a banner, button, or text link, has been exposed to a potential watcher. This is the amount of times that image appeared on any computer monitor anyplace in the planet.

Number of ADS you place on your website that is IMPRESSION
And how much visitor are responding on that ADS that is clicks

Impression is the number of times your impression is visible to the visitors and clicks is the number of times the visitors clicks on your impressions.