ZOMG a flip flopper! It's the over riding perception that a candidate is malleable, as opposed to flexible, were he or she to change a position.
At least I think that's what it is.
I think what people don't want is someone who is opportunistic, who changes their tune to whatever side will get them the most votes. That shows a lack of principle.
"Flip-flopping" can also seem negative as something that shows the candidate lacks confidence. It might seem to some that the candidate doesn't have strong ideas or convictions of their own, so they can be swayed easily. Leadership requires confidence.
I think it's good for someone to change their views in the face of new evidence or new perspectives, but I might be suspicious of someone who changes their views depending on how extreme their shifts were, or how quickly those views changed. For instance, Mitt Romney seems to attempt to hold almost all views simultaneously without committing to any of them. That makes it hard for me to trust him.