JSON is short for java script object notation. It's a type of object in js and also a file type that you may run into. It's VERY important in java script, but not as much in game maker. It's rare that you'll ever actually need it but you may need it sometimes if you need to import/export a file of this type.
JSON (JavaScript Object Notation) is a lightweight data-interchange format which is easy for to read and write, for both people and machines