Black's king is now boxed in, and will never advance to (or beyond) the marked squares in the diagram.
This illustrates an important point: it is not always best to check the enemy king. It is often best to instead trap the king and limit its movement. Remember that checkmate is the goal, not simply random checks.
Black must make a king move; 1. ... Kd7 is as good as any. How can White continue to make progress towards limiting Black's king?
