Here is a useful animation on the Behavioural Economics topic of nudging from the Rotman School of Management in Canada.
“A nudge, as we will use the term, is any aspect of the choice architecture that alters people’s behavior in a predictable way without forbidding any options or significantly changing their economic incentives. To count as a mere nudge, the intervention must be easy and cheap to avoid. Nudges are not mandates. Putting fruit at eye level counts as a nudge. Banning junk food does not.” Thaler and Sunstein
People don’t always act rationally. In fact, they tend to act irrationally – but in predictable ways. The concept of the nudge, involves making a deliberate change in choice architecture to engineer a particular outcome.