How to read the heap diff?

[Broham]

I have an application where on app start I am creating a HeapDiff and on the leak event I am performing a diff to see what is leaking. One item jumped out at me:

{ what: 'String', size_bytes: 4785072, size: '4.56 mb', '+': 32780, '-': 563 },

But I am unclear on how I should track this down to the code that is causing the leak. Is there documentation anywhere that can tell me where to look when "what" is "String"? There are also other item's I would be interested in tracking down like "Object" and "Code".