A symbol is normally a tangible, concrete item that represents something more abstract. For instance, in Call of the Wild, the man in the red shirt is wearing a symbol -- the red shirt. The color red can be used to symbolize a number of different emotions or signals: danger, rage, intense hatred, or even the idea of stopping, such as when we see a red light.
The red shirt is symbolic of the lesson learned by Buck at the hand of the man in the red shirt -- he is the man who beats Buck and other dogs severely using a club, and Buck learns "man's law" through the violence of the man in the red shirt, who is never given another name. Hence, the red shirt is symbolic of the above mentioned emotions -- rage, hatred, and intense distrust.
The club itself could also be considered symbolic, as it represents man's abuse and torture of animals in the Yukon. It isn't until Buck meets John Thornton that he is treated with the civility and respect he deserves. All his other masters, including Perrault and the others, may have treated him somewhat kindly, but not with the love and compassion that Thornton exhibits.