Gravity is the force with which bodies pull each other. This force or gravitation pull is given by Newton's law of universal gravitation, according to which the gravitational pull between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them.
or, Force = Gm1m2/r^2
Gravity is the reason why we can walk on the surface of the earth. In the absence of gravity, for example, in space, we will keep on floating instead of walking. The gravitational pull of the sun keeps Earth in its orbit, otherwise Earth would have moved out of the solar system (and it is the proximity to the sun that enables life on Earth). Similarly, the moon's gravity causes tides on Earth. Gravity is also the reason why everything we throw up always falls down (as long as its velocity is less than the escape velocity).
Hope this helps.