It's a tiny computer that you can stick into a TV or monitor like a Chromecast or Roku Stick, except it's a tiny PC instead. It's basically a small and rather slow computer that fits in your pocket and that you can connect to any display.
Such devices have been around for a long time, and I was initially surprised that they didn't become more popular. Then I realized that the portability gets offset by the fact you'd also need to carry your mouse and keyboard with you, together with their cables and dongles, and connect them whenever switching locations.
It becomes quite a hassle, and you might as well just get a tablet or 2-in-1 instead that has everything included and ready to go anytime.
If you don't need portability, you might as well get a larger, much faster and more reliable device. These sticks use old netbook quality chips and super-slow memory card system for storage. You wouldn't find such slow components in any other devices, not even mid-range phones.