A metasite is a grouping mechanism that allows you to combine data from two or more websites. You don’t have to export data from separate websites and calculate metrics manually, which saves time when analyzing data from multiple pages.
Get more information on metasites on the Piwik PRO help center:
Use metasites to merge data from websites
and on our blog: The New Piwik PRO Analytics Suite 10.0. – Create Reports Easier and Faster