In my experience people use "metaphysics" to refer to philosophical exploration of what kinds of things exist and what the nature, behavior, etc. of those things is.
This is usually treated as distinct from scientific/experimental exploration of what kinds of things exist and what the nature, behavior, etc. of those things is, although those lines are blurry. So, for example, when Yudkowsky cites Barbour discussing the configuration spaces underlying experienced reality, there will be some disagreement/confusion about whether this is a conversation about physics or metaphysics, and it's not clear that there's a fact of the matter.
This is also usually treated as distinct from exploration of objects and experiences that present themselves to our senses and our intuitive reasoning... e.g. shoes and ducks and chocolate cake. As a consequence, describing a thought or worldview or other cognitive act as "metaphysical" can become a status maneuver... a way of distinguishing it from object-level cognition in an implied context where more object-level (aka "superficial") cognition is seen as less sophisticated or deep or otherwise less valuable.
Some people also use "metaphysical" to refer to a class of events also sometimes referred to as "mystical," "occult," "supernatural," etc. Sometimes this usage is consistent with the above -- that is, sometimes people are articulating a model of the world in which those events can best be understood by understanding the reality which underlies our experience of the world.
Other times it's at best metaphorical, or just outright bullshit.
As far as correct behavior goes... asking people to taboo "metaphysical" is often helpful.
The rationalist taboo is one of the tools I have most enjoyed learning and found most useful in face-to-face conversations since discovering the Sequences. Unfortunately, it's not practical when dealing with mass-broadcast or time-shifted material, which makes it of limited use in dealing with the scenarios where I most frequently encounter the concept of metaphysics.
I tend to (over)react poorly to status maneuvers, which is probably part of why I've had a hard time with the word; it gets used in an information-free way sufficiently often that I'm tempted ...
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.