Structurally, heterochromatin is highly condensed while euchromatin is uncondensed or little condensed. Functionally, genes in heterochromatin are turned off due to their physical inaccessibility while those in euchromatin are able to be expressed.
Work Step by Step
This is covered in the middle of p. 330. The physical condensation of heterochromatin, unlike euchromatin, makes the machinery of transcription unable to access genes and, thus, it cannot begin gene expression. Think of a map-- if you wadded it up and wrapped tape around it, it would contain information which was inaccessible, like that in heterochromatin.