Nobody really knows what it means. If they do, they aren't saying. People have given theories about the meaning of that line, but there is no universally accepted answer. You should take into consideration, the time in which it was written, as well as the recreational drugs Jim Morrison used. He has a fan of psychedelics. If I remember correctly, one of the band members said that his lyrics didn't always make sense, because he wanted it to seem cryptic and mysterious. Unless you can bring Jim Morrison back from the dead to ask him, you'll probably never get an answer.