Found the answer to my own question. The Highlight plugin automatically (I think) adds that bar (which I believe it calls the "overview") to the right. Any other words in the buffer, or any visible buffer, that match the word at the caret get a mark in the overview.

I've found so many uses for it.

One of them is to be able to quickly navigate. If I'm at a function, I can put my caret on the function name, then look in the overview to see where on the buffer (or even another buffer in a split view) that function is being used. I then scroll to that mark on the overview (or I can just click the mark itself).

Another use is to see if a function/subroutine/variable/object is being called in another buffer (as one often has logic in one buffer and the view/display page in another). In a split view, I have the model page in one, and the display in the other. I click on the name of the function/subroutine/variable/object in the model pane, and if it is used in the display, the overview bar will show a mark. It's nice that I don't have to scroll up and down looking for it.