A generalized variable or place form is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the cars and cdrs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored.
Generalized variables are analogous to “lvalues” in the C
language, where ‘x = a[i]’ gets an element from an array
and ‘a[i] = x’ stores an element using the same notation.
Just as certain forms like
a[i] can be lvalues in C, there
is a set of forms that can be generalized variables in Lisp.