Foobar is a universal variable understood to represent whatever is being discussed.
It's usually used in examples that illustrate concepts and ideas in computer science.
For instance, a computer science professor may be discussing different file formats. In this case, he would call the generic-example file foo or foobar, then list the extensions associated with the file formats (e.g. foobar.txt, foobar.gif, foobar.exe, foobar.tar).
When foo or foobar is used, everyone understands that these are just examples, and they don't really exist.
Programmers and administrators also use foo and foobar in a similar context. Files or program s named with foo or foobar are understood not to be permanent and will be changed or deleted at anytime.
Foo, bar, and the compound foobar were commonly used at MIT, Stanford and the Helsinki University of Technology, Finland. Other generic variables are used other places, but only these three are considered universal.
One last note: hackers never use foobar to mean fubar!