The peak-end rule is probably something you’ve noticed in your day-to-day life. Stated simply, the peak-end rule is a psychological heuristic which dictates that “People judge an experience by the most intense point and the end point.” Psychologists Kahneman and Fredrickson tested this “rule” and elaborate on their findings in this paper from 1993.
To frame this idea with a contemporary example, think about Uber. One of the most forward-thinking elements of an Uber experience is that you don’t have to spend time fumbling with money or cards, deliberating about the driver’s rate or tip, you simply exit the car when you arrive at your destination. This leaves people with an overtly positive end to their experience. No hassle, just function. In this sense, the peak-end rule accounts for one factor which has made Uber (and similar apps) so popular.
So, the climax and the resolution of an experience – of ANY length of time – are what resonate the most. Why should web designers care? In short, because visiting a website is its own experience. People are guided along an arc of clicking, scrolling, reading, and viewing. In order to make a lasting impression, designers need to position important information and calls-to-action at a distinct climax and end-point. For example, if a user enters their contact information into a form on a company’s site, does another message pop up thanking them, and maybe prompting further engagement? Even for small interactions, the ideal is “a fantastic ending that leaves a strong, positive emotional imprint.” Keep this in mind for future web projects, and read some more examples on the mental phenomenon here.