Why do web developers use "FOO" all the time?

Is this some kind of inside joke that I'm not getting?

Maybe "foo" refers to me, as a foo, because I don't understand what foo means?

Oh look...it's even in StackOverflow page for asking questions:

How to Format

<http://foo.com>
[foo](http://foo.com)
<a href="http://foo.com">foo</a>