Stuff changes its motion when you push on it. If there is a lot of stuff, it doesn’t change motion very much.
Newton’s second law of motion describes how objects move when pushed. Newton’s insight was that a net force (that is, a force that isn’t matched by an opposing force) causes an object to change its motion but that not all objects change their motion by the same amount. If I push on a toy truck with the same force needed to move a real-life truck, their response will be very different indeed! The toy may well fly across the room while I may only just be able to budge the real truck. What’s the difference between the toy and the real truck that contributes to this difference? It’s their mass!
Newton’s 2nd Law can be summarized like this:
It takes more net force to get the same change in motion for more massive objects..If you apply the same net force to objects of different mass, the less massive objects will change their motion less.