A procedure invocation in Scheme is written like this:
(procedure [arg1 [arg2 ...]])
In this expression, procedure can be any Scheme expression whose value is a procedure. Most commonly, however, procedure is simply the name of a variable whose value is a procedure.
For example, string-append is a standard Scheme procedure whose
behavior is to concatenate together all the arguments, which are
expected to be strings, that it is given. So the expression
(string-append "/home" "/" "andrew")
is a procedure invocation whose result is the string value
"/home/andrew".
Similarly, string-length is a standard Scheme procedure that
returns the length of a single string argument, so
(string-length "abc")
is a procedure invocation whose result is the numeric value 3.
Each of the parameters in a procedure invocation can itself be any Scheme expression. Since a procedure invocation is itself a type of expression, we can put these two examples together to get
(string-length (string-append "/home" "/" "andrew"))
— a procedure invocation whose result is the numeric value 12.
(You may be wondering what happens if the two examples are combined the other way round. If we do this, we can make a procedure invocation expression that is syntactically correct:
(string-append "/home" (string-length "abc"))
but when this expression is executed, it will cause an error, because
the result of (string-length "abc") is a numeric value, and
string-append is not designed to accept a numeric value as one of
its arguments.)