Krittivasa is one among the numerous names of Shiva. Shiva is known as Krittivasa because he wears animal hides as clothes. Shiva usually wears a tiger skin. As per some scriptures he also wears elephant skin.
Symbolically, the tiger skin suggests that Shiva is the controller of all powers in the world.
A story is mentioned in the Shiva Purana, which explains why Shiva wears tiger skin. You can read it here.
When Shiva wears elephant skin, he is also known as Gajantaka. You can read the story and symbolism of Gajantaka here.
Another reason for Shiva using animal skins as clothes is that he is the lord of all creatures – Pashupati. He is the divine herdsman.