The molt, ecdysis, is a very delicate process the adult spider goes through once or twice a year – or more frequent if immature. When they are molting their skin, the exoskeleton, they re-new itself – lost legs and urticating setae are replaced. The young molt at the beginning of their lifes more frequently than the adults, about once a month and then more seldom. Before the actual molt the spider refuse food and burrow with the opening of the burrow sealed with silk to prevent insects to disturb it. Tarantulas are helpless in the process of molt.

Exuvium of Lasiodora parahybana

A certain sign of a upcoming molt is when the skin on the abdomen gets darker, especially visible on american species where they often have a bald spot. Other signs are refusal to feed approx a week (or longer) before the molt.

O
The spider spin a silken mat where she lay on her back when molting. The old skin cracks up due to the spider pumping out blood and she can begin to push herself out.

The abdomen of this Xenesthis intermedia getting dark.

When she released herself from the old skin she is vulnerable and sit still until the new skin has hardened. It takes up to two weeks before she eats again for a adult tarantula.

Recently molted juvenile. The spider will darken after a day or so. 

The young spiders usually deals with the molting process a bit quicker, and eats within a week after a molt.