It lays down in simple yet accurate terms the numbers that desribe how a force changes the motion of a body. The acceleration of a mass is directly proportional to the force applied to it and inversely proportional to the mass of the object. The second law can be considered the most important, as all of dyamics' basic equations can be derived from it using calculus. If more than once force is acting on an object, than they must all be added together to compute the equation. The magnitude and direction of every force must be combined to form the resultant force, which is used in the equation.
Acceleration is also a vector quantity and it is in the same direction as the force.
It may be more accurate to say that the actions of two bodies on each other are always equal in magnitude and opposite in direction. If you press your hand against a wall with a certain amount of force, the wall will push back against your hand with the same amount of force. Legend has it that Newton was considering his Third Law one day when he saw an apple fall from a tree. He noted that the earth was pulling on the apple and the apple was pulling on the earth. It turned out that he was right.