Actually the question seems to be very easy, but answer is never gonna be easy. This is a game theory question, involves game tree concept, backtracking concepts, some pruning rule like alpha beta pruning. There is lots of things you should know before you get answered.
By the way in simple way, suppose A(Computer) and B(Human) are two players, then how the game goes is depends on how the players are playing-
(3) Both Aggressive and Defensive
Now the calculations are done in a manner to get a score that tells you probability of winning after some move.
I will suggest you, study the concepts of Game Theory, then try to know the calculation otherwise forget about the question.