When a website appears as a Google organic (non-ad) search result, any traffic their page gets from that search is counted as an SEO click. We roll up an estimated number of clicks that they get from each keyword from the past month. This helps you determine how much traffic their web pages get-main page, blog, etc.
We estimate these clicks using information from Google and careful calculations based on how that company ranks on every one of its searches.
The chart of Est. Monthly SEO Clicks on the SEO Overview helps you see how the number changed over time and against Google algorithm updates.