Tag pane is a Pro-only feature that lets you see all your tags in one place.
You can open the tag pane like so:
In the tag pane, the list of tags in the current document and in all documents are displayed and dynamically updated. Clicking on each tag will initiate a search for that tag.
The tags are ordered by frequency, and the number of occurrences is noted in parenthesis right after the tag. You can change how they are sorted at any time like so:
You can also rename a tag like so: