Croatia has Just Joined the Schengen. But What is Schengen?

The Schengen area has announced its 27th member state. From 2023, people passing between Croatia and other Schengen countries will no longer need to be checked at the border. The Schengen area is the world’s largest open travel zone. It consists of 23 EU countries plus Lichtenstein, Iceland, Norway, and Switzerland. But what is Schengen?