They wrapped this standalone player into portable formats (like PortableApps.com packages) so you could carry it on a USB stick. It was useful for developers testing games, or for playing Flash files on work computers where you didn't have admin rights to install plugins.
Cybercriminals know that people are desperate for Flash. They create fake "Flash Player Updater" pop-ups and fake "Flash Portable" installers. These are trojan horses.
Avoid third-party "portable Flash Player" downloads. Use Ruffle or convert your old Flash content to modern formats (HTML5, WebGL) where possible.