Tarantulas have an exoskeleton. This means that they have a skeleton on the outside. It is just as good as a normal skeleton except exoskeletons don't allow the tarantula to grow. This is why tarantulas molt.
Adult tarantulas molt about every year but growing baby tarantulas molt often. Molting is a process where the exoskeleton of a tarantula is shed. This allows the tarantula to grow. The tarantulas go on their backs when in the molting period. This is the usual position. They pull their legs and also the rest of the body out of the outgrown skin. It will have a soft skin and it will be a bit shiny.
The tarantula will refuse food for at least a week and sometimes they refuse food for a month. It's not a very good idea to put food in yet because with the skin not hardened, even a cricket could kill the tarantula. A good sign to show that the skin is hardened is the fangs will turn from white to black.