Payam Fard wrote://select[not(//label/@for=./@id)]
It finds all of the elements whose name is "select", which are not in any namespace, and for which the expression in [...] can be coerced to "true".
The expression inside [...] is called a "predicate" and it should be an expression which can be coerced to boolean, as this one is. Ignoring the not() function, the expression is of the form A=B. Note that either side (or both) of such an expression can be a nodeset, and two nodesets are equal if any node in A is equal to any node in B. (At least that's how it is in XPath 1.0; it might be different in XPath 2.0, but that's beyond the scope of what I know.)
Does that get you started? The expression itself looks like somebody made it up as a joke, though.