Q+K+p vs Q+p

Hi everyone, I recently was looking at this position (White to move):

Here the engine says that White really only has one good move: Nd6 (+8.68 at 36 depth) compared to the next best move Qb8+ (+1.30 at 36 depth).

Can someone help me understand this position, and why it is so important to play Nd6 here? Any insight is much appreciated.

Since there are 7 pieces, you can use tablebases to find the best possible moves for both sides and that will give you some idea of the right strategies. Here's a link to the position on the Syzygy TB, which confirms 1.Nd6! is the only winning move. White wants to exchange the Qs and sacrifice the N for the h-P to win with the remaining P. Apparently the immediate 1.Nd6 aiming for g3 is needed to stop the h-P in time. 

Thnk you very much the to allow me play here .I want to play chess very much , but the line chess in my country is very bad .

The dog