An impression is a metric that lets you know how many times people saw your links (on search), ads, or content (on social media). It helps you to calculate the clickthrough rate (CTR) and engagement rate. You can use impressions in social media, SEO, and paid advertising.
CTR is a metric that lets you see the percentage of people who clicked your links/ads from the total amount who saw them.
Engagement Rate lets you see the percentage of people who engaged with your content from the total amount who saw it.
Let’s say your goal is to drive traffic to your site through paid ads on Facebook. You have lots of ad impressions, but very few people click. That means people aren’t interested. So to improve your CTR, you need to make your ads to make them more click-worthy.
Same with engagement rate; you usually want people to like, comment, and share your content. So if you have low engagement, you want to create more relevant and exciting content.