If something is justifiable, it was done for a good reason and is easily explained or defended. Your fear of gerbils is justifiable, given your experience with the mutant gerbil that ate your neighbor's dog.
Justifiable means something is "able to be made just," and you'll hear the word often in legal contexts. "Justifiable homicide," for instance, is killing someone in self-defense — a situation where an act that is otherwise criminal is "made just" by the circumstances. You can also use the word in a more positive sense. For example, you can take justifiable pride in winning the race even though you started off running in the wrong direction.