You use the clone brush to replace and object by another.
For example, you have electrical wires going across the top of your photo.
This looks ugly. You want to remove them. They go horizontally in the sky.
So you "clone" the blue of the sky over the black wires.
To do this, you click on an area of the sky close to the wire. You will get
a cross hair (+) where you click. Remove your finger from your mouse and
move the pointer, a circle appears (O). Then you move your the O close to
the wire, put your finger on your mouse and move the O over the wire. The
black wire is now replace by blue sky. As you move the O, the + also moves.
So you have to watch both. The O always replace (clones) according to the
area were + is.
So if you started in the blue sky, but then the + is moving over a white
cloud, the black wire is replaced by white. If you want blue, click on Set a
new position and click in a blue area of the sky and continue.
The above is fairly easy to do, a little trickier would be, say, removing
the strap of a bra over the shoulder of a woman. It looks ugly. Here you
would set a new position several times, replacing (cloning) skin close to
the strap as you move over the shoulder.
It is good to Set a new start position often, since if you make a mistake,
you just undo the last action.
It is a great tools and with a little practice you can remove objects
easily.
Another example would be an electrical receptacle on a wall. Here you would
click on the wall, close but not too close to the receptacle and move the O
over the receptacle.
Start cloning some small objects to get the feel for it, and the trick is to
always keep an eye on the location of + because this is the area you will be
cloning when you move the O.