Tag is a non-hierarchical keyword assigned to a report. Tag allows the report to be found again by browsing or searching. Tag is set globally and is visible for all user who have access to the reports.
Tags can be added directly from the report by hovering over the More... link.
Searching by Tags
Click on Reports in top menu. You can see list of all available reports. If the reports is tagged, the tag is also displayed. You can click on any tag to use it for searching. Or you can type directly tags:nameoftag to the search field.