If you hover over the number you'll get a short description of what the field is. There's more information in our blog post: https://itch.io/updates/updates-to-project-analytics-filtering-collections-impre...
In order to give you more visibility around how much exposure itch.io gives your project page, we're now showing Impressions and CTR on your analytics page. Unlike the other metrics, these only account for the last week of data. An impression is incremented every time itch.io shows your project title and thumbnail to someone when browsing our site. This includes pages like the homepage, the feed, recommendations, and our browse pages. The CTR, or click through rate, represents the frequency an impression ended up in someone clicking through to your page. (Note some pages may not have Impressions & CTR if the data is too low)