Definition
An actionable metric is one that ties specific and repeatable actions to observed results.
If you are an eCommerce site, you have a baseline of revenue, and you introduce a new website design for a portion of your website with improved images and more information about those products, and revenues for those products are up 20% compared to the same period last year, then revenue is an actionable metric. There is an obvious action: apply that new website design to the rest of your site.
Further Reading
Book: The Lean Startup by Eric Ries