Before finding out what Resolution Unit stands for, one has to understand what an image's or video's resolution is.
Resolution can be described as either the total amount of pixels in an image. In most cases, however, it consists of the width and height of the image. This also applies to the visual screen size of a video.
As mentioned above, the resolution might indicate the total number of pixels. However, there are other units than pixels that might be used for describing the resolution of an image or video. For images and even videos, this is typically pixels, but when talking about screens, for example, the unit of inches is far more common.
Thus, the resolution unit gives you information about the actual unit the resolution – which you will also find in the metadata of your file – is in.