Demos are typically released by the game's publisher to help consumers get a feel of the game before deciding whether to buy the full version and/or keep it.

What is demo of a game?

A demo is a limited version of a game intended to let interested gamers try it before purchasing. Demos usually offer a very limited window into the game—usually just a few levels or features.

What makes a good game demo?

A good demo is representative of the final game, quickly

When you start working on your demo, keep in mind that its most important feature should be that it's an accurate representation of what the final experience will be like. That doesn't necessarily mean that your demo should be the beginning of your game.