Queer

Queer 1: The short explanation is that queer is mostly used by people under the gay umbrella and or someone who is under the trans umbrella (mtf, ftm, nonbindary and more.)
Queer 2: Someone can also use queer as their sexuality (often used by people who are more comfortable with it or doesn't know what their sexuality it. If you want to know more about that then you can always ask the person themselves.)
Queer 3: To explain to someone that they aren't straight.

If you want to know what that person mean by queer just ask them. They should answer your questions if you ask them nicely and explains that you mean no harm in your questions. If they don't want to answer then they don't have to. Remember that they are still a human.

Queer 1:
1: Do you know their sexuality?
2: No but I know that they are queer.

Queer 2:
1: Okay so they are queer?
2: Yeah, they feel more comfortable with using queer than explaining their sexuality every time.
1: But how are they going to find someone if they don't tell them?
2: I actually don't know but I will ask them next time I see them.

Queer 3:
1: What do you mean you don't (only) like the opposite gender?
2: Well I'm queer.