Explain the significance of the expression "sign your name in blood" as it is used in this chapter

This happens when the boys crawl into a cave and Tom introduces the idea of

“Tom Sawyer’s Gang”. The boys must take an oath and sign their name in blood to join. This is an initiation process that Tom had picked up somewhere. It is his way of pledging loyalty and brotherhood to his gang. Tom feels this gives his gang a certain amount of credibility right from its inception.