Some apps can only be installed from their own repositories. That usually involves some faffing with aging let’s to your key store, etc. That’s what the cryptic instructions are about. Most apps can be installed with one click from the distro’s main app store. Ubuntu is somewhat restrictive here, because it forces you to use snaps, which is their own proprietary packaging format. Other distros, e.g. Mint are more inclusive here. This is why Ubuntu isn’t recommended as much any more.