Dear sir

I am a hobby programmer in germany. I write a chess programm to learn programming. And I do not understand this alpha beta system.
There is always only one move considered. And it is assumed when this move is better than the best before this move will not be chosen from the opponent. But I cannot understand this because there could be for example that the second player beat the queen. So the first player would not choose a move that enables the opponent to beat the queen. But sometimes it could be worthwhile to sacrifice the queen when some moves later more gain is possible. So with the alpha beta I would overlook this move.
But you stated that "while guaranteeing the same result as a full Minimax search of the same depth"
This is what I not understand