The idea here is to force black to move histheir knight. We can see that the king is already in a state of not being able to move because of the white king and light squared-squared bishop. And we can see that the dark squared-squared bishop on any of these highlighted squares would be mate.
So all we have to do is bring the dark squared-squared bishop to f2 and so forcing the knight to move. The knight (moves highlighted isin orange) cannot simultaneously block or defend bishop b6 or h4 mate at the same time, nor can it take the bishops or check the king to delay a mate in 2.