I tried to google it and it left me very confused.
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There are a few different explanations which are all looking at it a different way so let me try one of the less common ones. When wind hits a flat surface it pushes on that surface. If the surface is angled then it will not only push in the direction of the wind but rather push the surface at its angle. You may have experienced this when holding your hand out the open window of a moving car, angling the hand may generate lift upward or downwards. For a sailing boat this means that they can change the angle at which the wind pushes the sail by angling the sail in the direction they want to be pushed, it is however still limited to 90 degrees of the angle of the wind so you can still not sail upwind, only cross wind.
But a sailing ship have two “sails”, there is also the keel. It acts as a kind of sail in the water instead of the air. And you can angle the keel using the rudder. The same way that the angle of the sail determines the angle of the forces from the wind the angle of the keel does the same with the force of drag from the water. This gives another 90 degree change in force direction. When a sailboat is sailing upwind they are therefore angling the sails so the force of the wind push them mostly cross wind but also a bit downwind and then the keel is angled further so that the crosswind forces from the sail is converted to upwind forces. If the upwind forces created by the keel is larger then the downwind forces from the sail the boat will end up sailing upwind.
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