Working with Heron, especially beyond its basic usage, requires background in the concepts behind GeoExt, ExtJS and OpenLayers. The GeoExt website has some excellent coverage on these toolkits. See further below.
These are pointers on getting started with ExtJS with an emphasis on important concepts in GeoExt derived from ExtJS:
See the Ext Primer.
Essential OpenLayers concepts for working with GeoExt: