I guess it's when you feel like you don't want to feel the way you do, but the way the piece of fiction was crafted makes you feel that way against your will. Like, when there's no reason for an element to exist in a story except to provide an emotional response at the desired moment, and it's done in a way that's immediately noticeable.
Basically, if you can imagine the writer rubbing his hands and going "mwa-ha-ha what do I pull out of my ass and drop into this story to play these suckers' heartstrings?" that's manipulative. If you instead imagine the writer looking the purposeful, meaningful, organic elements of a story and going "oooooh, if I adjusted this a bit, it'd be ten times sadder than it already is!" it's not manipulative.
A lot of this is massively YMMV, of course. Plus, there's always some instances where you see a scene and you know it's manipulative, but don't really care or get offended because it still affects you.
Re: Can someone please explain to me
Basically, if you can imagine the writer rubbing his hands and going "mwa-ha-ha what do I pull out of my ass and drop into this story to play these suckers' heartstrings?" that's manipulative. If you instead imagine the writer looking the purposeful, meaningful, organic elements of a story and going "oooooh, if I adjusted this a bit, it'd be ten times sadder than it already is!" it's not manipulative.
A lot of this is massively YMMV, of course. Plus, there's always some instances where you see a scene and you know it's manipulative, but don't really care or get offended because it still affects you.