Received means just that - They have recieved it (posted or delivered by customer, paid for and entered onto the system and scanned for public viewing), but is awaiting further processing. Documents with a received status can and often are returned for amendments to be made.
Registered means that information contained on the form has been checked and if correct are then "registered" and the CRO Register is updated to reflect this - director details, registered office details, share/mortgage details and so on.
Documents that appear as Registered are locked and cannot be returned for any reason unless a High Court order is obtained.