Queer is an identity, and is often considered a movement, of people that fall outside of and/or reject of the cultural norms around sexuality and/or gender identity or expression. The word queer can mean different things to different people, but common definitions include, someone who isn't straight, someone who isn't male or female, or someone who rejects cisheteronormativity. The Queer Movement has largely been about anti-conformity and the rejection of binaries and being pushed into boxes.
Queer is often used as an umbrella term for those in the LGBTA+ community. It can also be used be as an orientation or gender identity in and of itself.