(hello "I'm a programmer." "Welcome to my homepage.")
More specifically, they are written in hyper-literate language. This means they are regular Racket modules that you can require or run. Each program’s page has a link to the program’s module in the ‘Source’ section. For example, to use the dataflow-variable class in your own Racket program, get the module dataflow.scrbl from the Dataflow Variables page and do (require "/path/to/dataflow.scrbl").
This section lists the module definition of the program. This homepage is also a program.
(define-syntax hello (syntax-rules () ((_ string ...) (for-each displayln (list "Hello," string ...))))) «greeting»