Once the enemy king has been pushed to an edge, it's important to make sure he stays there. We accomplish this by placing our queen on the "second line" - the rank or file next to the one on which the enemy king is trapped.
For instance, in this example, the Black king has been forced to the 8th rank. On order to ensure the king stays put, White correctly moves his queen to the 7th rank by playing 5. Qe7. Black's king is now reduced to shuffling between c8, b8 and a8.
