The midpoint is a part of the story that is more or less in the middle or before the climax takes place. The midpoint takes your main character back to basics. It takes it to a part in him or herself that makes it question everything he/she does. In this case, the audience will either also doubt the character’s solving problem abilities or will cheer for your MC so that the main conflict can be resolved.
Let’s do an example: Let’s say your character found some powerful alien technology and has overcome every obstacle with humans. The MC is confident that he can take out anybody. He knows he has to save the world. Now when the real threat comes forward who is another extraterrestrial with similar technology, he fails. Let’s top it a notch. The MC fails and it’s stripped from the technology. Not only the MC know he can’t win, he doubts he can’t do it with the technology by his side. How he overcomes the mental and physical obstacles brought to this part? Those are the questions that your audience wants to be answered. If you do this correctly it will hook again the reader.
The midpoint is necessary in order to put all the things in perspective. To remind the character and the reader what is at stake. When you have a standalone story this midpoint has to be critical but not as big as the climax. When you get a trilogy maybe you can get away for the first book that the midpoint is critical but not that big as you are pursuing to up the stakes with each book.
Sometimes writers struggle with filling the middle part of their manuscript. They know how it starts. They have that killer fight scene, that super climax that will resolve the main conflict… oh yeah the middle. That mushy part on the middle that is so blank. Go back to your outline or create a part, it doesn’t have to be right in the middle but between points A and B so you have like a checkpoint to were you have to arrive. It will be easier and now you won’t have a big hole in the middle, instead of two small ones that are easier to cover. You are a creative person so roll your sleeves and work on it.