From this tab, you may learn what common words appears on your organic competitors' pages. Surfer lists those words from the most common, to the least.
Look at section "9 out of 10 pages". There are two phrases: "content optimization" and "content marketing".
That means that when analyzing top 10 results for your keyword, on 9 of them those words appear. That may indicate that those phrases are important for Google bots and your audience and it's highly recommended to use them in your content.