What is a "frozen ATPL"?
A frozen ATPL holder is somebody who has passed all the required (14) exams for the issue of an ATPL but does not have the required flying hours. Strictly speaking, there is no such thing as a 'frozen' ATPL, though a number of training schools do use the term:

A Frozen ATPL holder refers to somebody who has passed all the required theoretical exams but does not qualify him to be issued with a full ATPL until he has logged a total of 1500 hours.
A student from a flying school will normally graduate with a CPL (Commercial Pilot Licence) and IR (Instrumental Rating) (CPL+IR). This Licence is known as the Frozen ATPL.

In order to 'unfreeze' the ATPL, the candidate has to complete a flight time of 1500 hours subject to certain conditions:

... ATPL will remain frozen until you have reached 1500 hours of flying time, which is made up of the following criteria:

500 hrs multi-pilot in transport or commuter aircraft
250 hrs PIC (or 100 Pilot in Command and 150 PIC under supervision)*
200 hrs cross country flights (100 as Pilot in Command)
75 hrs instrument time
100 hrs night flight as PIC or co-pilot