The Routing option in Add Result Block lets you define if this result block should be routed to a Content Search Web Part instead of being shown in the Search Results Web Part.
Any routing label that is defined as part of adding a result block will appear as options in the Result Table section of the Content Search Web Part.
Content Search Web Parts are mostly used in search driven sites, such as public-facing websites.
For more information, see : Create query rules for web content management in SharePoint Server 2013 (TechNet).
