Here’s the same position slightly earlier so you can see how it was created. White has nothing at the moment, so he plays with any checks he can inflict. He has two: Rd8+ and Qa4+. Rd8+ loses the rook, though it moves the king over and causes Black’s knight to become pinned—a good thing to see, but not yet useful. Qa4+ loses nothing, and so is of particular interest. Carefully consider Black’s possible replies: any ways he can interpose something between his king and White’s queen, move his king out of check, or capture White’s queen. Here Black’s options are three:

(a) Black can move his king to f8. White asks what checks he then would have, and sees Bh6. This drives the king on to g8. White asks what checks he then would have, and sees Qe8#. Conclusion: if Black initially replies Kf8, White mates.

(b) Black can interpose his knight on c6. When you imagine an enemy piece moving, always keep an eye on how this affects any lines it used to occupy. Here the knight’s departure from e7 would vacate the path of White’s bishop to d8—a square adjacent to Black’s king and already attacked by White's rook. White would be about ready to play Rd8#; the only hindrance would be Black’s queen. But it can be drawn out of position if White plays QxN+: for then if Black replies QxQ, White mates with the rook.

(c) Black’s final possibility is to interpose his queen at c6. Now analysis proceeds as in the previous frame, with White again looking always for his next check and this time finding Rd8+.

It is worth going over these variations until each of them—and the process of thinking all of them through—is clear.