This is about triangulation. Triangulation is a method in the endgame. You use it when it is a king vs. king and pawn endgame or similar endgame, when you have the advantage. You use your king to make a triangle (hence the name triangulation) and lose tempo to gain opposition.
In the example, if it were black to move, black would have to move backwards giving white the opportunity to go into blacks position and promote. However, because it is white's turn, white needs to lose tempo. See the diagram for more detail.
As you can see, triangulation is an important skill to know. For more information watch the video on triangulation by Jrobi!