Last Rank: This is the most recent organic position we saw for the domain on this keyword.
Best Rank: This is the best organic position (closest to 1) that we saw for the domain during the selected date range.
Cost per Click: The average amount advertisers pay Google anytime someone clicks their own ad for this keyword. The default is “broad match” costs, but exact match and phrase match are available on the results page for this keyword.
Ranking Difficulty/Keyword Difficulty: We’ve calculated how difficult it would be to rank on this keyword. The score is based on a scale of 0-100 (with 100 being the most difficult to rank for). Compare this number to other keywords you're targeting to get an idea of how to prioritize your SEO campaign.
Keyword difficulty takes into account the strength of the domains, on-page signals like “keyword in title,” and the number of .gov and .edu domains.
Monthly Searches: Shown elsewhere as "Volume," this shows the number of searches done this past month across the US on Google.com (or in the UK on Google.co.uk if you are looking at UK data). We blend data from multiple sources to give a better snapshot of search activity on this keyword, so it won’t be identical to Google’s metrics for search volume.
Est (POS) Clicks/Mo: We estimated the number of possible clicks that this domain gets in a month from ranking on this particular keyword.
Est Click Value/Mo: This helps you estimate what ranking on this keyword means--in dollars--to this domain. It provides a more meaningful reflection of what SEO performance brings to the bottom line.
It is an approximate value of a keyword, similar to buying ads on a keyword via PPC. This gives a fair value to organic rankings so you can estimate how much they return and prioritize the keywords you aim for. The SEO value mimics PPC costs, but with some adjustments.