Function CLOSE is a GENERIC-FUNCTION, see Chapter 30, Gray streams.

When the :ABORT argument is non-NIL, CLOSE will not SIGNALs an ERROR even when the underlying OS call fails.

GET-OUTPUT-STREAM-STRING returns the same value after CLOSE as it would before it.

CLOSE on an already closed STREAM does nothing and returns T.

If you do not CLOSE your STREAM explicitly, it will be closed at garbage-collection time automatically (with (CLOSE stream :ABORT T)). This is not recommended though because garbage-collection is not deterministic. Please use WITH-OPEN-STREAM etc.