Simple Definition:

A Newton is a unit of measurement for force [a push or pull]. It is named after Sir Isaac Newton, who discovered the laws of motion. One Newton is equal to the force required to accelerate a mass of one kilogram at a rate of one meter per second squared [increase it’s rate of movement by one meter per second, every second].
For example, pushing a book weighing one kilogram with a force of 5 Newtons would make it accelerate at 5 meters per second squared [every second, it’s rate of movement would increase by 5 meters per second].

Very Simple Definition:

A Newton is a way to measure how hard something is pushed or pulled. It is named after a scientist called Isaac Newton. If you push something that weighs one kilogram with a force of 5 Newtons, it will start to move faster by 5 meters per second every second.