The 'Plugin Bar' is a panel at the top of the Vuze interface, hidden by default, that contains a number of views containing small amounts of information that users may wish to have displayed along with the usual Vuze components. It was originally intended for use by plugins, hence its name, but it also contains core views so would perhaps more appropriately be known as the 'Top Bar' (compared to the Side Bar and Status Bars).
The Plugin Bar can be shown/hidden by using the F8 function key (similarly to the way the Side Bar can be shown/hidden via F9). There is also a menu option under the 'View' menu to do this.
By right-clicking in the view list area (on the left) it is possible to selectively show/hide the individual views. An advantage of hiding unwanted views is that the scroll bars required to show all views can be removed, effectively de-cluttering the display.
Shows a scrolling graph of overall download speed
Shows a scrolling graph of overall upload speed
Some users requested quick access to some configuration settings and features - this view contains a few such options along with a handy 'temporary speed limit' control that allows you to, say, throttle Vuze to low network utilization for a few hours while you play a game and then have it auto-revert when the time limit is exceeded.
For those user interested in knowing which ISP/VPN they are currently using this area shows the current IP and ASN details.
Since version 5741 this area shows the most recent un-viewed notifications. Clicking on it switches to the Notifications view.