The "subsidy" (Einspeisevergütung) is a fixed price paid for power sold to electricity providers, for those who don't want the complexity of taking part in the energy market. It was supposed to accelerate the transition to renewable electricity production by making financing more predictable.

And it worked very well, as it started the global solar boom. Because it's not necessary anymore as an incentive, it's slowly getting phased out.

The Einspeisevergütung is not the reason why consumer electricity is expensive in Germany, though. That's more related to our grid.