Falsifiability is very important in science; scientific theories and hypotheses can not be scientific unless they are falsifiable. When creationist finally learn this they attempt to use it to discredit the theory of evolution.
Before I tear apart the argument I should explain what falsification is and its relevance in science. If something is unfalsifiable it can not be shown to be false, if it is falsifiable is can be shown to be false. Science does not prove theories and hypotheses, when scientists test theories and hypotheses, they test them in an attempt to falsify them. This is why falsification is so important in science.