A ''not negotiable cheque'' means that if the cheque is transferred to another person, the person who obtains the cheque has no greater rights to it than the person who gave it.
This is simply a warning to whoever has been given the cheque, to verify and ensure that the credentials of the transferor (former owner) of the cheque are intact without defects or irregularities. Because in case the title of the transferor is defective, then the tranferee also gets the same defective title