This is a generic term for someone in the LGBT community. All it means is “not CIS/straight.” It’s a catch-all way of saying, “everything else.”
I got the definition from a friend at work who is queer and non-binary. They said:
Queer is a big ol blanket term that basically means “part of the LGBTQ+ community”. It’s used flexibly for folks for a number of different reasons.
Some folks use it because they don’t want to label their experience but still identify as LGBTQ, or some folks may be questioning what they are and so go with the blanket term.
For others, the way they identify is complicated and may not be very well known or recognized even in certain LGBTQ circles, so queer is easier and accessible. It can be used as nebulous sexuality or nebulous gender.
Some, like myself, use it to mean that they are a participant in LGBTQ+ culture – or queer culture. Because some folks are LGBT but aren’t active in the community or culture, this distinction gives a sense of how closely folks identify.
It’s also a lot faster to say than most things other than gay.