ACTS when the word is used to designate timing in a story, refer to commonly accepted phases of a story line. These phases and what is in them have been observed over tens of thousands of successful stories (ones people like) in print and in film; and when we find stories people do not like, we often find they have violated the act structure.
In general the structure is in four parts. "Act I" introduces major characters, including the villain, even if the audience does not know it is the villain yet. This is typically in the first 15% to 20% of the story; if you are watching a 90 minute (of acting) movie you have probably seen both your hero and the antagonist in the first 20 minutes. You also have to introduce the setting, and any major suspensions of disbelief must happen there: You can't get halfway through the story and suddenly introduce magic. If there is going to be magic, or aliens, or at the end of your story your hero wins using martial arts or shooting a man from a block away, you need to introduce such "improbable" skills in Act I. She is wearing a black-belt in a Dojo, finishing up her daily exercise; she and her instructor know each other.
Even if your hero does not know magic exists in her world, your audience must. Even if your hero does not know an asteroid is going to wipe out Earth, your audience should.
"Act II" is usually broken into two parts, each about 30% of the story. The center of Act II usually contains the Major Turning Point: In most plots things are getting progressively worse for the hero until then, and may continue to get worse, but around the half-way point when everything seems lost and defeat seems imminent, something happens, or is discovered, or a decision is made that is the key to the hero's success.
(The opposite plot still follows the turning point idea: Things get progressively better for the hero in Act I, until "happily ever after" seems inevitable, but the Turning Point transpires and eventually means the undoing of everything.)
The second half of Act II is marked by progress, often with continuing bad stuff that is the ramifications of what has already happened: People are still dying, the villain is still powerful, the hero is still struggling and losing assets and people but she is making progress in some way, the audience is in suspense and rooting for her, but it still seems the hero can be lost. The villain or his henchmen are crazy skilled and smart and it seems the hero is barely a half-step ahead of him (but do not make her too lucky to escape, or introduce a new knife-throwing skill or ability to instantly hypnotize hotel clerks).
The Third Act is the explosive finale. The hero finds or accomplishes the last piece of the puzzle, knows how to solve the problem, and "locks in" the solution. You have to tie up any big loose ends here, but it can be done briefly.
Usually villains are defeated in the last 5% of the movie; and any scenes after the defeat are very short, tying up a single significant plot point introduced in Act I, for a kind of closure with the beginning of the story. This is the hero's reward, in essence: He kisses the girl, or beats the bully that was picking on him in Act I (the one you introduced to show how weak and pathetic he was). In The Pelican Brief, Denzel and Julia are successful and safe (respectively).
In essence, anything after the defeat of the villain is to briefly suggest how the future unfolds from here, and prove the success was permanent. This often involves a jump into the future; which can be hours, months or even years, depending on the scope of the hero's dilemma. The jump may be necessary to ensure plausibility of new emotions being expressed: After the harrowing run from assassins and horror of murders all around her, it would be ridiculously implausible for Julia to be happy and relaxed on the beach a few hours after she and Denzel finally won their battle. It would take months for anybody to believe they were finally safe, and that is the jump made.
The Act structure, and what story elements belong in each Act, are a shorthand story professionals use for typical, commercially successful stories; in a way they condense human psychology concerning stories and how we (on average) expect them to unfold, what we find suspenseful and fascinating, and what to avoid to prevent boredom or confusion.
Minor violations certainly exist; but you cannot have no conflict or villain until the last 15 minutes of a film. It is hard to imagine a good movie in which the hero does not appear within the first five minutes; usually they are in the opening scene. And so on; basically the Acts are about the touchstones and landmarks of a good story. Violations are (to me) usually cheats, laziness, and detractions from the story; to me it usually means the author wrote themselves into a corner and couldn't figure a way out, so they used some sort of magic or something bordering on a secret super power (ninja level martial arts) or some implausible coincidence (Sheila, to her alarm, finds a loaded gun on the floor of the bus, and for the safety of children puts it in her purse, intending to turn it into the police...).
To me the definition of a good writer is that they can follow all these rules and still come up with a fun new story; they have imagination and can put a character in a situation that truly does seem hopeless for them, hanging by a thread, yet plausibly still get them out of it in the middle of Act II.