If all moves that a player can play weakens his position but this player has to play a move because it is his turn , it is called Zugzwang. The player has to make a move and he is obligated to play one of the bad moves .
Black is in zugzwang. Black must make a move and to move his Knight will worsen his position:
White pawn will promote after 1. ... Nf3 ( or Nh3) 2.h7 or
1. ... Nh7 2.Kxh7 Kxf5 3.Kg7
The situation of White is zugzwang. White cannot prevent to lose a piece. a3,a4 and b3 are temporary moves.
Even if he plays these moves, he is forced to play one of the moves given below later .
1. Bc1? BxN : White lost the Knight.
or 1. Re2? (or Rgf1? ) BxR : White lost the Knigh.t
or 1.Rc1? Re2 2. Qxe2 Bxe2 : White lost the Queen.
or 1. Bf1? BxB : White lost the Rook.
or 1.g4 R(f5)f3 2. BxR Rh2 checkmate