When several forces act on an object, they can be replaced by a single force with the same effect. This is the resultant force. For forces along the same line, add forces that act in the same direction and subtract the smaller from the larger when they act in opposite directions. The resultant acts in the direction of the larger force.
A skydiver has a weight of 800 N downwards and air resistance of 300 N upwards. The resultant force is 800 - 300 = 500 N downwards, so the skydiver speeds up. The same ideas apply to an isolated solid object or to a whole system, such as a car and its trailer: find all the forces acting on it, then work out the resultant.
If the forces are balanced, the resultant force is zero. An object at rest stays at rest, and a moving object keeps moving at constant velocity, which means the same speed in a straight line. A car with a driving force of 600 N and drag of 600 N has balanced forces and zero resultant force, so it travels at steady speed.
If the resultant force is not zero, the forces are unbalanced and the velocity changes. The object accelerates in the direction of the resultant force. It may speed up, slow down or change direction.