Can some explain how DecodeJSON works?

JSON is a format for storing JavaScript objects.

These objects have strings as keys and values which are strings, numbers, booleans, or other JSON objects.

If you build a table with strings as keys, then it is equivalent to a JSON object.

JavaScript has a standard of representing these objects in strings that can be encoded and decoded. ROBLOX's decode and encode functions do this; essentially it means

encode(someTableObject) is someString;

decode(someString) is someTableObject