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.