Why do examples always use foo and bar? What do these two mean

  node.js, question

as the title says
Why do examples always use foo and bar? These two are What Do You Mean?

In the relevant documents of computer programming and computer technology, the term foobar is a common anonymous pseudonym and is often used as a “pseudo variable”. Technically, “foobar” is likely to spread through Digital Equipment Corporation’s system manual from the 1960s to the early 1970s. Another argument is that “fooba r” may come from the inverted foo signal in electronics; This is because if a digital signal is active at low level, a horizontal horizontal line will be marked above the signal mark, and the English word for the horizontal line is “bar”. In the new hacker dictionary, it is also mentioned that “foo” may have appeared before “FUBAR”