The “severity” column shows High, Medium and Low warnings for each resource that is blocked. Your goal is to make sure that you are not blocking any resources from GoogleBot or that any blocked resources have low severity status.
Here is a screen shot of the warnings from my account. Notice that most the resource warnings that are blocked are low and from third-party ads or scripts on the site:
John Mueller of Google said on Google+, where he announced this feature, that blocking resources “sometimes plays a big role in how Google’s able to render & index a page for search.”