'Queer' has two possible meanings that I'm aware of. One is the meaning of 'strange.' 'My stomach feels a little queer today' means 'my stomach feels a little strange today.' 'Queer' is also a general term that describes someone who is not heterosexual or not cisgender. Cisgender means that you identify with the gender that you were born with. It is sometimes used as an insult, but I've both heard of and met people who self-identify as 'queer' so that's not always the case.

Queer has a couple of uses. It can be used to say that you are not feeling well 'I'm feeling queer' but this is less common these days. It can also be used to say something is strange or unusual. The final common meaning is generally regarded as an abusive term to describe someone who is homosexual - not recommended to be used as it can cause an undesirable reaction.

It's almost exclusively used as a derogatory term for homosexuals. At least where I'm from.

Nowadays, it's often used as an umbrella term for someone who isn't CIS but also doesn't identify as a different label (gay, lesbian, etc.). BUT, some members of the LGBTQIAA community hate the term because it was used as slur, while others peacefully use it to define themselves, so I think the etiquette, when talking with someone who belongs to the LGBTQIAA world, is to ask them if they are ok with you using the term, It also can be used as an homophobic insult.