Degression is a new word to me, and maybe to you?
It means that the Solar PV starting Feed In Tariff is to go down each year to reflect the fact that installation costs are going to come down, and thereby keeping the financial return constant. The tariff will still be linked to the RPI but each year will start at a lower level for new entrants. The full degression table can be found here.
A useful guide to all tariffs can be found here.