Set Up

By the end of the first part of your story, you need to have introduced the main character(s) and what he/she/it/they want (or at least what "direction" they seem to be headed in, even if the specifics of what they want isn't 100% clear yet).

There needs to be some sort of movement toward a goal of some kind. Once the movement starts, the story starts.

Often there will be an inciting incident which starts the story moving.

Usually you will have presented some idea of the kinds of (known) obstacles that will be a problem in achieving the goal. One does not simply walk into Mordor...

Often you will have presented the kinds of help that can be expected, in moving toward the goal.

Usually you will have introduced a home/safe/familiar setting they start out in, which setting they will most often have to leave, in order to achieve the goal, or perhaps returning to which place will be the goal, or part of it. In fact, often the first act of the story ends precisely when they leave the familiar "home" setting.

Trying for the Goal, Failure and All Seems Lost

In the middle part of the story, new settings are explored, with new, unexpected dangers and complications. Things are even more doomed and complicated and impossible than anyone could have ever suspected.

Often, helps that were supposed to be there are not there after all, and supposedly helpful people may die, vanish, leave, or turn out to be betrayers. Special items will be lost or broken or will fail everyone.

Lots of little failures will happen, causing doubt.

By the end of this act, perhaps once there is some false confidence, things usually go horribly, horribly wrong. The story seems to be have been broken. It looks like nothing could possibly work out satisfactorially now.

Often the hero appears to be dead, the charming guy is engaged to someone else, and/or the goal is likely never going to be achieved. Sometimes the hero gives up, or is powerless. Other times he, she or it has sacrificed himself heroically and all seems lost.

A fancy word for the "lowest point" is the "nadir."

Resolution

Things need to be resolved, now. Heroes return from apparent death or unconsciousness, while Jedi and Kings Return, and so on.

Usually, this is the part (if it's an action movie) where the music rises, the hero gets back up, and the climax and catharsis occur. If it's a romance, the music rises and the happy-crying and crying-hugging starts again at this point.

In either case, all of the tension that's been built up to a peak is released and the audience feels the sudden, satisfying relief they've been waiting for after all the stress.

All prophecy, forshadowing, secrets, destiny and irony need to pay off by the end, though not everything must be fully explained.

Once this happens, there is generally a denouement, a small wrapping of up loose ends, so the audience can leave the book/theatre/movie/show without too many annoying questions having been left hanging.

A return to the setting and situation from the beginning of the story may show how much the main character has changed.