Threes are noted in all forms of culture. Films, books and plays usually have a first act, second act and a third act. In every good novel, a plot is naturally divided into chapters, but it is usually possible to pinpoint the moment when the beginning gives way to the middle, and then the middle becomes the end.
At the beginning of your novel, on the first page, you need to hook readers with something happening, or at least with a character worth caring about. There should be dialogue and readers should be exposed to what the protagonists goal is.
Once you have them hooked, you take readers on a joyride giving them “ups and downs” like a roller coaster, a ride for their money. Once you have established the intent of your protagonist, which you do at the beginning of the story, it goes into the second phase, what Aristotle called the rising action. The action clearly grows out of what happened in the beginning. (Cause and effect.)
However, the protagonist runs into problems, which keep them from successfully completing their intention. Aristotle called these barriers reversals. Reversals cause tension and conflict, the engines of a good plot; because they alter the path, the protagonist takes to get to their intended goal.
After the reversal, Aristotle suggested something he called recognition, which is where the relationships between major characters’ change because of the re-versal. That recognition is the irreversible emotional change within the characters brought about by the event. Both the reversal and recognition come from the story being told, not from out of the blue. (No Deus ex Machina aka God from the Machine.) A plot device whereby a seemingly unsolvable problem is suddenly and abruptly resolved by the contrived and unexpected intervention of some new event, character, ability or object.
Readers want to keep on reading your book. They want to be entertained, even more than educated. Readers want to stay with you to the end. So give them a payoff. Give them an ending that makes the whole trip worthwhile. If the ending doesn't deliver, readers feel cheated by the entire experience. The Payoff is the satisfying ending.
The ending is the logical outcome of all the events in the first two phases. Everything inevitably leads to a final resolution in which all is exposed and clarified. Everything is explained and everything makes sense. The best, most satisfying ending resolves everything, even the questions reader might not have thought to ask. You should leave your readers feeling complete.
Your ending need not move them to tears, although there's nothing wrong with that. You don't need to leave them feeling happy, although you can. However, the conclusion should leave them knowing they've witnessed a great fight, and the fight is over. They should feel the issues raised have been addressed and have been resolved. If the ending doesn't deliver, readers feel cheated by the entire experience.