This illustrates the most common way to combine expressions. To the left of that is h , the variable we just bound to a string value. Then there's string-append , a function that concatenates strings. An important thing to note here is that string-append is just another variable, like h . The only difference is that h is bound to a string value, while string-append is bound to a procedure value.
Procedures are just another Scheme data type, like strings or numbers.
As you can see, this is a very simple process, called procedure application . Don't be intimidated by the use of terms like procedure , arguments , variable , bound , and self-evaluating expression to describe it. I only introduce those terms here to help you read Scheme documentation later. Not every Scheme expression works by procedure application. At that point, h was not defined yet and could not evaluate to anything.