Well, JSON is the abbreviation for "JavaScript Object Notation". It is a text-format, representing a subset of JS which is useful for storing information in an ordered fashion. It is very simple to use, and since it is JS, any JS interpreter can simply interpret it, which makes it ideal for web apps (any browser can interpret JS, right?) Another important fact is, that it is very easy to read as a human (in contrast to binary formats). Its simplicity allaround has made it the #1 format for web data exchange.
Alternative formats could be XML (which was very popular before JSON and still is important in anything .NET), YAML (which is quite important in the Java world), TOML (which is widely used in the Rust world) and many more. You should choose your format based on your requirements; for example if you have a lot of table data and work with Excel, but want to display the data in some web-app at some point, csv might be a better choice.