The marshmallow test was developed in the 1960s and children who participated in it have been followed for over 40 years. It's about self control and willpower. Follow up studies showed that the kids who were more able to wait (15 minutes) ended up performing better academically and mostly continued to show self control in other experiments in later life.
This video is very cute to watch and gives you some idea of the process.
Read a Time Health article about the marshmallow test here. And a New Yorker article here.
