It's about cause and effect. Usually it is used for large systems such as weather prediction etc. And it points out that when modelling a system that not matter how small, even the beating of a butterflies wings [only used cause the idea is, it is such and small and feble creature] can dramatically change the outcome of a system, when followed through.
And the movie butterfly effect is relevent because it shows factors in his life changed one by one and over time had dramatic effect.. or something like that..