An AppImage is an app package for Linux distros that is an alternative to Flatpaks, Snaps, and apps packaged for traditional repositories. They are different from these because they do not need to be installed on a distro.
Rather, the package contains the app and all the files needed to run it. It does not need to use any libraries from the Linux distro on which it is running.
Some Linux versions of apps are only available as AppImages.
LibreOffice’s AppImage comes with all the apps the full suite offers. There are several different choices that can be downloaded from https://www.libreoffice.org/download/appimage/
The apps can be stored on a USB drive or another type of external drive or virtual drive and taken from computer to computer.
Here are a few apps packaged as an AppImage. Clicking the links will download the app.