Tarantulas are invertebrates in the arachnid family, and like many of their close cousins, they have an exoskeleton. That's a skeleton outside your body. We're mammals; our skeleton's inside our body, surrounded by flesh. We don't molt, but we do flake off our skin. Our skin constantly comes off our body as our body grows.
So, imagine a tarantula, whose skeleton is outside of his body. As the insides are growing, what is he going to do? He can't just shed off a little bit of skin. He's got to actually come out of that old skin. It's a shell surrounding his body. So, nature has designed the molting process, which is a painful experience, I believe, for the animal, in that he has got to pop out of his old skeleton, make a hole somehow to get out of it, and come out and start life anew again. So, why do they molt? It's because they're growing. That's a good sign, to see them molting.
Young spiderlings, or slings, will molt every couple of weeks, every couple of months, depending on the species, because they're constantly growing. Adult tarantulas very, very infrequently will it happen, but as long as they're growing, it should happen. It's a way for the animal also, if he's in the wild, to get parasites off of his body, anything that would be infecting his skin on the outside. If he's a tarantula that flicks his hairs to defend himself and has a bald spot, when he comes out of that molt, he's fresh and beautiful and clean and new, much like a snake after he sheds his skin. Here's an example of a molt of a rose hair tarantula.
That came off of him. It's an exact replica of his body, but it's light as a feather because it's nothing but the external layer of his exoskeleton. So, they need to do that. It's a good thing for them. It tells me, as a keeper, you're doing the right thing. Molting is good. Molting means growth. If your tarantula is molting regularly, you're doing a good job.