Tagbar
What?
It's more commonly known as "tagcloud", but pImgDB's version is a list in the sidebar. Click "Tagbar" in the main sidepanel, or select it in the dropdown menu top-left in the window.
Using Tagbar
The tagbar will be refreshed automatically when you click the "Tagbar" button in the main sidepanel. You can also refresh it manually by clicking the text up top. Whenever you refresh the tagbar, it will collect all tags from all the images you are currently viewing; the number next to each tag is how popular that specific tag is amongst your images.
Whenever you click a tag, pImgDB will perform a search for that specific tag in your database. If you now refresh the list, it will be populated with tags from that search result. That way you can jump from tag to tag, and easily skip through your database.