It generally means broken in some fashion. The word has a universal negative tone but can mean different things depending on context like a lot of slang.
The moving parts don't quite fit together in the way they feel they should. It is exceedingly difficult (perhaps impossible) to play the game in such a way that the action looks good on screen. Controls are perhaps unresponsive, or ill-suited to the gameplay situations they accompany. The visuals are of uneven quality.
There's loads of things that say "jank" to me. It's kind of a catch-all phrase for "this game has technical or mechanical problems."
I define it as something that is less than optimal. Something that doesn't feel smooth, or top-of-its-game comparatively. However, jank holds certain appeal, and the roughness can sometimes be beneficial. They can have charm, and with it, a certain spirit or mood, that may go against what is 'beneficial' generally, but make in turn benefit the project in question.
An awkwardness that doesnt render a game unplayable but noticeably imperfect. Not always a bad thing because in time it becomes part of that title's personality and charm.
Janky doesn't just have to be a feeling, it can also encompass a general sort of bugginess, but minor bugs that you can tell they didn't want to bother to fix...maybe you can't even say they're bugs for sure, but the game certainly seems like it's acting wrong. Could be physics bugs. Could be some weird interface quirks. That sort of thing.
I've only heard of "janky", which is like getting something new but it's already broken. Like if you buy a janky chair, you sit on it and it's already wobbly or squeaky.
Unpolished, rough around the edges, usually in relation to control/movement or animation. First example that pops into my head is a modern third person game where jumping is floaty or awkwardly timed, with an animation that jarringly transitions into jumping without any feeling of weight.
I think a number of things need to be off-kilter to meet my definition of jank.
Stiff animations, poor hit/collision detection, propensity for particle flaws, clipping, ill-timed music/dialog cues, just a general looseness with properties and mechanics, etc.