This is a way to see how much value a site could credit to its SEO performance. Every keyword has an estimated SEO value, very similar to how keywords have an average cost per click value on the PPC side.
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We look at all of the domain’s keywords and where it ranked for each one. That helps us figure out how many clicks it gets from each one. With the value and the number of clicks, we can roll up all of that value into a monthly figure. Just like how we show you the site’s estimated AdWords ad Budget, its SEO counterpart is this Estimated Monthly SEO Click Value — a rollup of all of its organic terms and their collective value that their clicks bring in.
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The chart of Est Mo SEO Click Value on the SEO Overview helps you see how the number changed over time and against Google algorithm updates.

You can search the domain’s SEO keywords list to see its ranking value for individual keywords.