Before determining the meaning behind what Resolution Unit stands for, it is important to know the meaning of what an image's resolution is.
Resolution is the total number of pixels within an image. Most of the time, however, it's comprised of the height and width of the picture. This is also true for the size of the screen in videos.
As previously mentioned, the resolution could refer to the total amount of pixels. But, there are different types of units other than pixels that can be used to define the resolution of a film or image. For video and images it is usually pixels, however when speaking about screens, as an example the measurement of inches is the most common.
This is why the resolution unit provides you with information about the unit in which the resolution is, which you will also see inside the metadata for the file that it is stored in.