The book is called "Call It Courage" because courage is what the main character, Mafatu, has to develop.
In the beginning, he is afraid of the ocean, which is a big problem for someone growing up in his society. So finally he gets up the courage to go out in a canoe to the other island.
While he is there, he does all sort of things (like fighting with the shark, the octopus, and the wild boar) that force him to gain confidence and courage. This is especially true when he has to deal with the cannibals.
So it is called that because Mafatu is gaining courage through all his experiences.