This quote, as noted above, is from Henry VI, Part 3, act III, scene 1. Crowns are an important image in this play, concerned as it with civil war and who will, ultimately, end up wearing the crown (ie., ruling the kingdom). 

As the quote suggests, Henry is a kind, gentle man, in over his head as he deals with a group of ambitious and ruthless people all vying to win at any cost. Henry might be a good man, but he is ineffectual as a ruler in this violent, treacherous environment. We might love him for having his crown in his heart rather than in outward flashy show and for defining his crown--what matters most to him--as "content," but contentment is not going to help him against people ravenously hungry for power. It will take a stronger ruler than this particular Henry to unite a kingdom being savagely torn apart. 

Shakespeare's quote, "My crown is in my heart, not on my head; Not decked with diamonds and Indian stones, Nor to be seen: my crown is called content: A crown it is that seldom kings enjoy" is a powerful one.  It indicates much in terms of the wealth of kings and the spiritual wealth of men, a theme seen in King Lear, amongst other Shakespearean works.  Of particular note in the quote is the contrast between Indian jewels and that of contentment.  Also, note the difference between physical wealth and spiritual gain.  The quote is located in King Henry VI, Part III, act III, scene I.  It is spoken by King Henry VI to the second keeper towards the end of the act.