Question about 'this'

[Joel via Digitalmars-d-learn]

        string getString2(in string input) {
                long start, end;
                while(start < input.length && input[start] != '"')
                        start++;
                start++;
                end = input.length - 1;
                while(end > 0 && input[end] != '"')
                        end--;
                end--;
                return input[start .. end];
        }

        unittest {
(543)           writeln(getString2(`st"1 2 3"`));
        }

control.d(543): Error: 'this' is only defined in non-static 
member functions, not __unittestL542_3

What does it mean about 'this'?