A plot twist is an unexpected turn—or twist—in the sequence of events that outline the plot. Some authors use plot twists to get a reaction from their readers or to completely throw off the readers from conventional way they expect the story to unfold. It leaves the reader saying, “I did not see that coming.”
This usually happens near the climax of the story, but can be found anywhere in the story really. A good example of this is how frequently the tides turn in Tolkien’s The Lord of the Rings. One moment everything is fine, then it’s not. One moment the protags are in the lead, and then the antags have the benefit. You just never know what’s going to happen haha. It leaves the reader on the edge of their seat, trying to guess what may come next, but the plot twist makes it impossible for them to foresee what will happen because of its unexpected nature.
The most impactful plot twists happen when the author does something unconventional to the protagonist (or sometimes antagonist). For example, at the end of George R.R. Martin’s A Game of Thrones (WARNING SPOILER AHEAD), GRRM kills off Eddard Stark, the noble protagonist that everyone loves.
You think, ‘Oh no, that character is fine because he’s the protagonist, which GRRM replies, “Nope!”
Another example is from the CW show The Vampire Diaries. (SPOILER AHEAD IF YOU”VE NEVER SEEN IT!) You think your main character, Elena Gilbert, will never become a vampire because she has established she never wants that for herself, and in fact, choses death over becoming a vampire, but in a twist of fate, she happens to have vampire blood in her system (a key component for humans turning into vampies when they die) when she dies…and then wakes up a vampire, which brings on worlds of new problems and conflicts.
So, to conclude, plot twists can be a good and effective tool for both shocking the reader and keeping them hooked longer to force them to find out the events to come :)