'Kill the pig, cut his throat, his blood.' - what is this line all about ? please explain.
It's an indication of how the supposedly civilized boys are turning into outright savages. There's an element of fear in their incantation; after all, this isn't something they've ever done before. The boys are stranded on a remote desert island and the pig represents the terrible fears they all share about their strange, new environment. In killing the pig the boys are confronting those fears; in performing their victory dance they celebrate their overcoming.
But there's a dangerous foreshadowing here: the boys are positively intoxicated by the spilling of blood; they actually enjoy it. The hunting and killing of a pig is no longer an act of brute necessity, a means of obtaining food; it's taken on a ritualistic connotation that binds the boys together with bonds of blood and common endeavor. Without even knowing what follows we sense that it is only a matter of time before it's not just the blood of animals that will be spilled on the island.
The boys have become enamored by the idea of hunting, particularly because they begin to feel that it gives them some control over all the unknowns. It is a way for them to try to conquer their hunger, their lack of purpose, and their fear of the beast all in one fell swoop.
The dance they do to mimic the hunt is their opportunity to draw up the courage that will allow them to actually kill, something that even Jack shies away from at the outset.
This line itself may very well invoke that urgent desire to strike back at their fear, to kill it, to drain it of its blood which gives it life.