An hierarchical grammar is a form of grammar where the expansion of terms can include (simpler) instances of the same term within them. For example, most computer languages, allow arithmetic expressions including ones consisting of two (simpler) expresssions with "+" between them: and also an expression surrounded by brackets:
expr ::= expr '+' expr | '(' expr ')' | ...
This allows complex constructions such as (1+2)+((3+4)+(5+6)). Note that regular expressions, whch are a simpler form of grammar, cannot match brackets like this.
Used on Chap. 14: page 328