When checking your files for hidden metadata, it may be that you find a value for something called Y Resolution. This meta information can show up in images and videos in particular. Yet, other files that have some kind of visual representation, for example, documents can contain information about the Y Resolution as well. As the name suggests, the Y Resolution hints at the resolution of a file, however only to the one of the y-axis.
In a system of coordinates, the y-axis is also known as the vertical or secondary axis. For a file that contains a value for the Y Resolution, this value defines the vertical resolution. This value is, in most cases, given in pixels. However, it may also be represented in cm (centimeters) or mm (millimeters).