First Act = 1st ~25% of film
It is the Introduction which sets up all of the important elements of the film, notably the problem to be solved, the protagonists and antagonists, themes, etc.
2nd Act = middle ~50% of film
It is where the Complications occur. The problem of the story takes twists and turns and gets harder.
3rd Act = last ~25% of film
Resolution. Wherein the problem of the story is directly and finally addressed and is overcome or fails to be overcome.
That's a simple overview.