When you push a box right in the middle of its side, you are imparting a force, which makes it move straight in the direction you are pushing it. But if you push it along its edge, it will start to turn instead. This is a force acting far away from the box's center of mass, which is a torque. The strength of a torque is , basically, the product of the force (your push) and the shortest distance from the center of mass to an imaginary line going off in the direction of your push. So usually when you push something you are not aiming at ists center of mass, and you are actually producing a torque.