How do you give feedback for an Interview?

When I finished an interview a few weeks back she asked for feedback. I didn't necessarily understand what that meant initially because I've never been asked to do so. I did mention it to my coworker that they requested feedback and she was just as baffled as I was. To give feedback is to be honest so how could I give them negative feedback? Is a possible way to do so without offending the interviewers? Or to even be respectful of the company who you're interviewing with?