Specifically, the three-act structure allows the audience to follow a clean and clear narrative. In the first act (introduction), what the film is about is defined usually in reference to what the protagonist’s role is in the story. The second act (complication) introduces the conflict that the protagonist has to try to overcome. The third act (resolution) is supposed to resolve the conflict and conclude the story. For a standard two-hour movie, the first act is approximately 30 minutes, the second act 30 – 60 minutes and the resolution 30 minutes.
Each act peaks at its respective plot point (with two plot points and the climax in the third act). A successful plot point ends the act and escalates the story even further by complicating matters even further. They essentially act as transitions between the acts by asking a question that the other act promises to answer.
In the example of the movie Finding Nemo:
The first act ends when Nemo is kidnapped by underwater divers and Marlin starts his journey through the sea in order to find his son. Thus the conflict is introduced.
Along with the conflict being introduced, another key character, Dory, enters the storyline as she tags along with Marlin as he tries to reach his goal of finding Nemo (hence, the title of the movie).The entire second act is composed of Nemo trying to get back to the sea and Marlin trying to find his son as he encounters sharks, jellyfish, and other perils of the big blue ocean.
The climax in the third act is when Marlin finally reaches the dentist’s office but sees Nemo upside down (implying that he is dead). He leaves, thinking that his son is dead.
However, Nemo is actually alive and finds his way out of the dentist’s office through the pipes and gets released into the ocean.
After Nemo is trapped one last time, the conflict is resolved when he finds his father and they live happily ever after.
In summary: The first act introduced the characters (Marlin and Nemo). The second act introduced the conflict and complications that arose as the characters tried to overcome the conflict (Nemo getting kidnapped and his father Marlin trying to find his son). The third act start with the climax and ended with the resolution (Marlin thinking that his son is dead goes back home but finding out that his son is actually alive and being reunited with him).
(Screenshots taken from the movie Finding Nemo)