Defaults refer to what happens if a person makes no choice or goes with a pre-selected choice. The influence of defaults is a foundational component of behavioral economics.
Perhaps the most famous example of defaults is the difference between opt-in and opt-out organ donation programs. While not universal, several studies have found that the rate of organ donation consent in a population seems to be influenced by the default (i.e., what happens if a person does not check a box or change the pre-selected preference on a form).
Smart defaults do not only refer to one-off events, however. In the well-known Save More Tomorrow program, participants were not only included in a savings program by default, but the amount they saved was also changed over time automatically (again by default). Similarly, other behavior change programs have default settings that include at-home medication or food delivery, rules-based reminders on different platforms, etc.