'Nudge theory' comes from Richard Thaler/Carl Sustein book Nudge: Improving Decisions About Health, Wealth, and Happiness. It is based on the idea that we are inherently lazy and tend to take the default option in most circumstances. The 'nudge' is to gently push us in the right direction for our benefit or the general good. One example would be making automatic kidney donation the default - you would need to carry an 'opt-out' card.
The central question, however, is who is doing the nudging? The government? If so, do you trust them to look after your best interests?
Interesting discussion of the issues in this BBC Radio show Analysis.