Who is pan and why is grover searching for him percy jackson

Pan is the god of nature. And Grover is trying to gain his searcher's license, to follow the tradition of his ancestors. That tradition is going on the quest to find Pan.

Two thousand years ago, Pan knew that his realm was being destroyed. He told a satyr named Lysas to spread the word that the great god Pan was dead. Mankind and other species believed this and began further destroying the places of the wild. Satyrs, however, never believed that their god was dead and began searching the world for the god of the wild. Because of their faith, Pan was unable to fade, but would sleep for long periods of time waiting for a satyr to once again take his message out into the world.