The author suggests that we consist of two systems. System 1 and System 2. System 1 is fast and System 2 is slow. We always use System 1 and sometimes System 2. Why? System 2 is lazy. System 2 is "rational". Not everyone likes to think harder about the problem. If something is not so important for us, then we just use System 1; it is just our "experience". I would compare this system to "pattern recognition", it is fast, but that's it. Sometimes we need to go deeper, so System 2 is needed. This system is more "rational".
We also consist of two selves: the remembering self and the experiencing self. The remembering self does the living and the experiencing self keeps score and makes the choices. In most experiments, we neglect the duration of an event. We remember peak moments and the end of the event.
We are not rational, so we should help ourselves using algorithms.