An AppImage is a way of
bundling the application with all the required libraries so that a
single AppImage can be run on wide variety of Linux distributions and
versions, similar to a disk image on macOS.
Why is there no AppImage for MKVToolNix?
In the past I didn't have plans for working on creating an AppImage, for several reasons. However, there is currently an effort underway by friendly collaborators to bring AppImage support to MKVToolNix.