Reading from /dev/urandom produces an infinite stream of random bytes.

The tr command filters characters from /dev/urandom: it deletes characters that don't match what is specified in its argument (a-zA-Z0-9~!@#$%^&*_()+}{?></";.,[]=-).

Then the fold command wraps the output every 32 characters and head takes only the first line, effectively stopping the pipeline after 32 characters were printed.