An impression is an event where a user views an ad or a link to a web page. Each view is counted as an impression regardless of whether the user clicks.
The percentage of clicks from impressions is how click-through rate is calculated. An SEO's concern is to make sure every webpage that appears in the SERPs gets as many impressions as possible and that each impression has the best chance of attracting a click.
Web pages that rank on the first page in the search results gain the most impressions. To get here, you need to make sure the page is optimised properly for the keyword you want it to rank for.
To attract a click, make sure your meta title and description are well considered. Badly written copy that doesn't meet the intent of the searcher will be less likely to attract a click. Keep an eye on impressions and click-through rates in the Google Search Console to identify pages that have a low number of impressions or a high number of impressions but a low click-through rate. More often than not it's possible to improve this by tweaking the copy in the meta title and/or description.