It's the flying you do between PPL and CPL. If you get a JAA PPL in minimum time of 45 hours and a CPL flying course cannot be commenced until you have 155 hours total, you have to build up your hours, preferably by getting good and relevant flying experience.
One would also need to do the same between completion of CPL and starting an FI(H) course for which one needs 250 hours.
Cheers
Whirls