It's a term that many would say is intentionally general.

Some use it as shorthand for "LGBT+" without being as cumbersome as an actonym.

Others use it to label themselves because they're unsure of where they fit but they know (or are at least pretty sure) they're not cisgender and/or not heterosexual.

Still others use it as s descriptor for either themselves or other people on the LGBT+ spectrum who don't fit neatly into the common categories.

Important addendum: it's not common nowadays, but "queer" was and is used in some circles as a slur. There's been an active effort to reclaim it, and it might feel a bit old-timey to hear it used with malice, but there's a reasonable minority of LGBT+ people who aren't a fan of being called "queer". There's nothing wrong with using it or with not liking it either, it's just something to keep in mind.