A free body diagram shows a single object on its own, with an arrow for every force acting on it. Each arrow points in the direction of the force, and its length shows the size of the force. A longer arrow means a bigger force.
Often two or more forces act on an object at once. The resultant force is the single force that would have the same effect as all of them together. Forces acting in the same direction add together. Forces acting in opposite directions are subtracted from each other, and the resultant acts in the direction of the larger force.
Example: a car has a driving force of 900 N forwards and a drag force of 400 N backwards. The resultant force is 900 − 400 = 500 N forwards, so the car accelerates forwards. A skydiver just after jumping has a weight that is much bigger than the air resistance, so the resultant force acts downwards.