Starting island is a small island where the PUBG game begins and players wait before going on the aircraft. Can be any random area of the map which was selected for playing.
While waiting for all players to join the match,you can run around, jump from one object to another, throw snowballs/fruits, use emotes, attack other players, etc.
Also, you can always return to the starting island after getting off the aircraft by swimming or riding a boat.