A clothes iron (also flatiron or smoothing iron) has a handle and a flat triangle shaped surface. Electricity makes the flat surface hot. A hot iron is rubbed on clothes to make them flat and smooth. This is called ironing. The handle does not get hot because that is where it is held. They are named clothes irons because they used to be made out of the metal iron.
Cloth is made from lots of small strings. When people wear clothes or move other things made from cloth, the little strings might bend and make lumps. Because the iron is heavy and hot when it is turned on, the little strings stretch and go flat when the iron is put on top. Different kinds of cloth need to be ironed in different ways. If the iron is too hot then the cloth might get broken or burn. If the iron is too cold then the strings might stay lumpy and not go flat. Sometimes people use water with an iron, which makes steam.  This can make it easier to iron clothes because the small strings get soft. Some cloth does not need to be ironed because the strings always stays flat.