Here, it is said that after 1.Re7 Rb8 2.Kd3 a6, white plays b4 to fix the enemy pawns. But I don't understand: how does it fix pawns? How is b3 any different? What does it mean to fix enemy pawns?
- b4 fixes the black pawns a to c, because they cannot advance without support.
In the original position, 1. b4 would not fix the pawns. Black could play ...b6 followed by ...c5 at the right moment. After 2. ...a6, ...b6 is not possible anymore without support. And without ...b6, ...c5 is not possible. So the black pawns need support by their King or Rook to advance. They are fixed.
I know the term „fixed pawn“ only in situations where the pawn cannot advance even with support. P.e. w a5, b4, c5, b a6, b7, c6 the pawns are fixed. It is obvious for the a and c pawns. If the b-pawn advances, it will be captured, and any recapture by a rook (a recapture by the king is obviously illegal) will result in serious material loss.