For one thing, the Law of Club and Fang is the title of Chapter 2. But beyond that, it is what Buck finds when he is taken away from Judge Miller's estate.
When Buck meets the man in the red sweater, he learns about the law of the club. He learns that a man with a club is likely to be willing and able to force Buck to do his bidding. This kind of treatment is unlike any he has ever seen before.
When he gets to Alaska and is put among the other dogs, he finds out about the law of the fang -- whoever can fight will be able to enforce their will.
So this law is basically the idea that the strong will control the weak. There is no more civilized behavior where Buck is now -0- the strong rule and the weak must obey or die.