The Glossary Viewer Tab Bar is similar to what you can see in most Web Browsers nowadays, where you can browse several URLs using separate tabs.
In the Glossary Viewer you can open several glossaries in parallel.
Each open glossary gets its dedicated tab, labeled with its name. You can easily switch back and forth between tabs by pressing respectively Ctrl Shift Tab and Ctrl Tab.
Clicking the button lets you browse and select a glossary, opens that glossary, and assigns it a new tab.
Clicking the button closes the current tab.
Right-clicking a tab opens the following context menu:
When you have more tabs open then can be displayed, some additional buttons will be shown: