The gravity of your planet will affect the development of your life forms. Gravity will hold onto your atmosphere and will affect atmospheric pressure. It will determine how hard raindrops hit the ground as they fall, so it will affect rates of erosion. It will determine how much force it will take to lift water up into your trees so that leaves will live. It will determine how heavy blood is, and how hard hearts will have to contract to make the blood circulate.
Gravity is the force that causes objects to attract one another.
Scientists are studying it, and they can describe the way that it works. However, no one knows what it is or why it works as it does. This power can be accurately calculated. Look at the picture of the circles above. You can see that the little gray circle in the center has a radius of 1.