$Failed
is a special symbol returned by certain functions when they cannot do what they were asked to do.
Examples
Basic Examples (1)
Opening a non-existing file is an operation that returns $Failed:
Wolfram Research (1991), $Failed, Wolfram Language function, https://reference.wolfram.com/language/ref/$Failed.html.
Text
Wolfram Research (1991), $Failed, Wolfram Language function, https://reference.wolfram.com/language/ref/$Failed.html.
CMS
Wolfram Language. 1991. "$Failed." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$Failed.html.
APA
Wolfram Language. (1991). $Failed. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$Failed.html