UN Goodwill Ambassador is a position traditionally held by movie stars (or maybe other celebs). A famous example was Audrey Hepburn. That is, I think, what they do- go to places and have their photos taken with refugees and people in need, to attract attention.
However, I don't think Jolie's clueless about what she does. She definitely doesn't have qualifications of more serious UN employees, but I saw an interview with her by George Stephanopulous (if that is how you spell his name), called "Miss Jolie goes to Washington", and she appeared clued in and informed. She's obviously educated herself a lot.