Unlike some languages like Pascal, functions are regular data objects and can be stored in data structures, extracted, and the result of a computation can be invoked like any other function
Note that the use of the term "function" does not imply anything in the sense of purely functional languages. The term is used here as in Dylan and C; functions can have side effects, and can return zero or more values (unlike C and Java which only support returning zero or one values)
Some languages, like C, support first-class functions -- function pointers, in their terminology -- but functions in RScheme are richer structures that in C because here they include their lexical environment. They are closures rather than just code.