Please explain the law behind gravitation force

You don't say what kind of law you're asking about, nor
what grade you're at, assuming this has to do with a science or physics class.  So, I'll
try to be as clear as possible.  For another explanation, see the reference
below.

First, you can ask "how does a body near the Earth
respond to gravity?"

The body -- whether a big rock or a
small feather -- will accelerate at a specific rate, about 9.8
meters per second per second.  This means, as Galileo demonstrated, that a "light"
object will fall exactly as fast as a "heavy" one.  Of course, you know that a feather
will glide, not fall like a rock: this is because air resistance is much greater for the
feather than the rock.  If you dropped the rock and the feather inside an airless
chamber, they would fall equally fast, and your brain won't believe what it
sees.

The second question you might ask is "why does the
gravitational force decrease with distance?"  Also, you might ask "How would I describe
the motion of an object that responds to the force of gravity?"  I'm assuming you're not
asking this, so I won't answer it here.  If you want to know this, and you understand
algebra, I can say more.  I'll check back in a little
while.

In case you aren't asking why gravitation decreases
with distance, skip the rest of this answer, which gets into a bit of
algebra.

Light and gravity happen to behave similarly in
this respect.  Here's a thought experiment: imagine your flashlight gives a square beam
(perhaps it has a square bulb?) that spreads out more as you shine it on things farther
away.

Suppose this light beam exactly covers one square
foot of cardboard, when the cardboard is 3 feet away; that is, it covers a square 1 foot
wide and 1 foot high.

Now, move the cardboard twice as far
away (6 feet, although the number itself doesn't matter).  Now the square beam spreads
out twice as far on each side, so the area of the square is 2 feet x 2 feet: 4 square
feet.

So, by doubling the distance, you have multiplied the
area by 4.  You have multiplied the distance by 2, and the area is multiplied by 4,
which is the square of 2 (the amount by which you
multiplied the distance).  In fact, if you multiply the distance by any number
n, you multiply the area by n*n.  This
relation is called a "square law effect".

But the
flashlight hasn't gotten brighter or dimmer: you have the same amount of light spread
over a greater area, so the beam looks dimmer.  In fact, since the area has been
multiplied by 4, the brightness has been multiplied by
1/4.

And this works for any distance also: if you multiply
the distance by n, you multiply the brightness by 1/(n *
n).  This is called an "inverse square
effect".

Almost done, catch your breath.  Gravity also has
an inverse square effect:  if you multiply the distance by n, you
multiply the gravitational force by 1/(n *
n).