It simply means that the holder of the license has passed all the theoretical exams at ATPL level, but does not yet have the flying hours for the grant of an ATPL (i.e more than 1500 hours, plus a whole lot of other requirements w.r.t the type of flying experience).
Now once they have the required aeronautical experience their license is 'unfrozen', i.e. they go to the CAA, give them a stack of cash, convert their blue CPL to a green ATPL, and then they are then able to exercise the full privileges of the ATPL license.