A force that causes objects with mass to attract one another.
Gravity is a force that causes objects with mass to attract one another. The more mass an object has, the stronger its gravitational pull. This is why, on earth, what goes up must come down. You are not really falling down but being pulled back towards the earth because the earth is much larger than you and therefore has a much a much stronger gravitational pull than your own body. This is the same reason the moon rotates the earth and the earth rotates the sun. The earth is stuck in the suns stronger gravitational pull and so is rotating the sun and the moon is stuck in the earths stronger gravitational pull and so rotates the earth.