The act of molting is a usually stressful process that invertebrates go through to grow. Since they have a hard exoskeleton they literally have to break free of their old skin, revealing a new skin underneath. At the time of molting the new skin is still soft and pliable and once blood is pumped to all of the extremities, the animal becomes larger.
In the case of tarantulas this is a fascinating process. Not only do they grow larger, but any hairs that were flung off in self-defense are replaced. If a leg has broken off, this too is replaced. After a molt, the tarantula’s colors are also very bright and fresh. Unfortunately though, this is when the animal is most vulnerable as it cannot defend itself.