App remnants are literally remnants or leftovers (directories or files) from apps that are no longer on the device. In almost all cases they can be safely deleted.
Note: Any utility that finds app remnants may flag some items as remnants even though the app is currently installed. That occurs even in the vaunted SD Maid app (which allows the exclusion of such false positives from the scan).
Case in point: When I use MiX's App Remnants feature or SD Maid, some files for a particular app appear in the scans but I opt not to delete them. If those items were deleted my device would probably still be okay but because those items are associated with a presently installed app I don't delete them. The point being, make sure to review the results of such scans carefully to make sure you delete only items for non-present apps.