This juvenile Brazilian salmon pink bird-eating tarantula (Lasiodora parahybana) is beginning her molt. Notice in the photo, she is laying on her back, upside down as she pushes the old skin off. As tarantulas grow, they shed their skin or molt. This process can take several hours as she eases her way out of the shed skin. The white or lightest part of her is her new skin while the darkest part is what she is shedding.
In this photo you can see her as she finishes the molt. She is still on her back and she has used her legs to push her way out of the molt. This time and energy intensive process leaves her tired and vulnerable for a few days because her skin is weak, soft and sensitive.
Once completed, she takes a few minutes to let her new skin dry and harden slightly before standing; and then she flips herself back over. She will not eat for a few days after this and will mostly hide out in her enclosure until she no longer feels vulnerable and is back to her normal self.