Lesson
White to move. Main line: 1. Qd8+ Kh7 2. Qxc8
After 1.Qd8+ the king has only one square — 1...Kh7, using the luft made by the h6-pawn — and then 2.Qxc8 snaps off the undefended knight. The check buys the tempo that makes the second threat unstoppable.
A double attack (or “fork”) with the queen is one of the most common ways to win material: give check, and let the same move hit a loose piece somewhere else on the board. Undefended pieces on the back rank are prime targets.