Wind is the movement of air. The reason air moves is because of a pressure gradient. A pressure gradient is caused by one location having higher pressure than another. Changes in air pressure are caused by the heating of the Earth, and in the case of wind the uneven heating of the Earth. So if one location gets more heat because it gets more sunlight (tropics) than another (mid-latitudes) the area that gets more sunlight (and therefore more heat) will have higher pressure. The will try to move from the area of higher pressure to the air of lower pressure, and this is the basis for wind.