The greater the difference between the low pressure above the wing and the higher pressure below the wing, the greater is the amount of lift produced. In order for all this to happen, air needs to be moving over the kite. When we fly kites we use a string to hold on to the kite, and to prevent it from being blown away by the wind. Birds move through the air by flapping their wings, while powered aircraft use an engine to push the air towards the back of the plane fast enough to make the aeroplane move forward. The force that makes something move forward through the air is called thrust .
Birds use muscle power to develop thrust, and except for gliders, aeroplanes use motors.
A kite cannot produce its own thrust, and instead must rely on being held in place while the wind moves past it. So the thrust that acts on a kite is produced because the kite is being held in one place by its string while the wind flows around the kite. If there was no wind a kite would only fly if the person holding the kite string started running, making their own wind as air started flowing around the kite. As the air flows over the surfaces of a kite the wind gets held back a little bit by the roughness of the fabric and the sticks. This friction is called drag.