Sir Isaac Newton first published his three laws of motion in 1687, in 'Philosophi Naturalis Principia Mathematica,' Latin for "Mathematical Principles of Natural Philosophy," more commonly known simply as 'Principia Mathematica'. His three laws describe the relationship between the forces acting on a body and its motion.
Newton's first law, also known as the Law of Inertia, states that a body will continue in its current state of rest or of uniform motion unless acted upon by external unbalanced forces. Uniform motion here means moving in a straight line with constant velocity.
If you place a ball on the ground in a football field, it will stay where you put it until someone kicks it. The kick gives it the force it needs to get moving. On the other hand, if you put the ball on a ramp, it will roll down. This time the force is coming from gravity.
If you're playing with marbles, and one marble is moving across the floor in a straight line, and you hit it with a second one, that deflects the path of the first one. It changes the state of the marble so that it is no longer travelling in a straight line.
The force(F) being applied on an object is equal to its mass (m) times its acceleration (a). To put it mathematically, F = ma.
Alternatively, the force on an object is equal to the time (t) derivative of its momentum. Momentum is the product of the object's mass and its velocity (v). Mathematically, F = d(mv)/dt.
When an object applies some force on another object, the second object simultaneously applies a force on the first one. The second force is equal in magnitude but opposite in direction to the first one.
This law is popularly summarized to say that every action has an equal and opposite reaction.
This law explains why a gun recoils when you fire a bullet from it, or a raft moves away from the swimmer when the swimmer jumps off it. Or why your hand feels sore if you slap someone.
Obviously, the recoiling gun does not jump out of your hand and fly off with the same kind of velocity as the bullet. The second law helps explain this. The force on both the gun and the bullet is the same, though in opposite directions. That means that the product of mass and acceleration is the same for both. Now the gun's mass is much larger than that of the bullet, which effectively causes the gun's acceleration to be much less than that of the bullet. So it stays put in your hand.