Coined in 1957 by Leon Festinger, cognitive dissonance refers to the uncomfortable feeling people get when one thought, action, or belief is inconsistent or incongruant with another thought, action, or belief that the person has.  When this happens, people feel compelled to reduce the dissonance by supplying additional information (justifying); disregarding or downplaying some information; admitting their thought, action, or belief was wrong; or changing their thought, action, or belief. 

An example is someone who refers to themselves as healthy but then eats cake at a birthday party.  This person may say that they worked out extra that morning (justifying); say that it's just one small piece (downplaying); tell everyone they know cake isn't healthy but they're going to eat it anyway (admitting wrong); or admitting they aren't healthy (changing belief). 

How does this relate to changing behaviors?  Over time people develop habits to meet their needs.  Some of the habits are maladaptive.  Then we ask or tell them to do something different.  This creates cognitive dissonance and people often fall back to what they know because of their need for certainty, even if it yields sub-optimal or outright bad outcomes.  

Due to this giant gap between their current habits and adaptive behaviors, change takes time.  This means that it is going to take a lot of persistence and patience on the part of teachers and parents to help our kids develop adaptive behaviors.