He is considered important because early in life he described the three laws of motion. The first law states that to make an object move, you need to apply a force - or, more correctly, an unbalanced force. My teacup is sitting on the desktop. It is motionless. Because all of the forces on the cup are in balance.
Gravity is a force pulling down on the cup, and the desktop is a force pushing up on it.
Gravity would pull it downward. There would be no desk top to push up on it. Gravity is an unbalanced force on the cup. Newton understood when all existing forces on an object are in balance, the object will remain motionless, or inert. The First Law of Motion is about inertia.