Data stores are a service offered to developers which allow them to save data and fetch it from any server in the universe (game), and are essentially structured as associative arrays.
Data stores are structured similarly to dictionaries/records. They consist of key-value pairs. Each key is unique and acts like a "header" or "identifier", if you like, and can hold a value that can be converted or serialised into JSON. This means that you cannot explicitly set a key's value to a userdata (which includes but is not limited to Instances), and can only save primitive types (numbers, booleans, strings and tables containing only these types).