When you're through using Scheme interactively, you need to be able to get out of it. You give a command to tell the interactive Scheme system (which is just a program) to terminate.
Most systems have a special command (starting with comma or whatever
the convention is), like ,exit. (It might also be ,quit,
,halt, or ,bye.) There may be a Scheme procedure you can
evaluate to kill the system, by evaluating a procedure call expression
in the normal way, e.g., (exit), (halt), (quit), or
(bye).
In many systems (especially under UNIX), you can use an interrupt
key sequence to kill the system, if you're at the top-level. E.g.,
at the top-level prompt, <ctrl>-D, may do it.