When you install with a package manager you download a collection of files. Many of these files are not the program itself but are required to run the program you want, they are called dependencies. When downloaded this way all dependencies are visible to all programs. A lot of different programs use the same dependencies but sometimes different programs will need different versions of the same dependency. This can cause issues where a program tries to use an incompatible version and it's called dependency hell.
Flatpack, snap, and appimage try to bundle the program you want with its dependencies in an isolated environment that only the program can see, so when another program that uses the same dependencies runs it cannot use the ones located in the isolated environment.
Flatpak and snap allow some shared space to avoid downloading the same files over and over again. Appimage does not and each appimage contains a completely isolated collection of everything need to run the program.