A presentation is a graphical view of an object which is directly linked to the object in some way. The help article links you see in the documentation browser are presentations; and if you see a word in the UI listener, all words in the definition will themselves be presentations.
When you move the mouse over a presentation, it is highlighted with a rectangular border and a short summary of the object being presented is shown in the status bar (the summary is produced using the summary word).
Clicking a presentation with the left mouse button invokes a default operation, which usually views the object in some way. For example, clicking a presentation of a word jumps to the word definition in the UI browser.
Clicking the right mouse button on a presentation displays a popup menu listing available operations.