The word "gravity" refers to the force that pulls an object to another object. More specifically, it is the force that the Earth or another heavenly body exerts on people or objects, pulling them toward it.
All planets have gravity, but some have more than others. After the sun, Jupiter has the strongest force of gravity in the solar system because it has the largest mass of all the planets. Gravity keeps people and things grounded on the surface of the Earth instead of floating through the air.
The word's origin comes from the Latin stem word "gravitas," which means heaviness. Another use of the word "gravity" means seriousness, as in the sentence, "He tried to explain the gravity of the situation to those who thought the prank was funny."