How to Understand Newton's second law

In this tutorial the author explains the concept of Newton's Second Law of motion which states that force is equal to mass times acceleration. So the equation to compute force F is F = m * a, where m is the mass of the body and a is the acceleration produced by the force on that body. He explains that even our weight is our mass times the gravity. So now he explains that mass of a body is constant and only our weight changes depending up on the situation we are in. If you want a clear understanding of the Newton's second law of motion, watch this video.