This addon modifies the mission tracker on the right side of the screen. When you hover over the current mission icon or text, you'll get a tooltip with the flavor text of that mission's current tier. Additionally, if there is an associated image, video, or audio clip, clicking the icon with show it. Note that this means you won't be able to click to open the Mission Journal any more - you'll need to use the "J" shortcut key.
Source code is available via git from curseforge.
If possible, please try to use the official forum for discussion in order to consolidate potentially useful information in one place.