All invertebrates must molt in order to grow. Invertebrates will refuse food several days before a molt. Most tarantulas will turn on their backs to molt. Don't panic if you see your tarantula lying upside down and motionless, leave it alone. Molting can take several hours.
Moisture is a very important factor in the molting process.
You can mist the sides of your tarantulas enclosure in addition to having a water dish at the time of a molt. After your tarantula molts it will be considerably larger then before.