Queer is a word that describes sexual and gender identities other than straight and cisgender. Lesbian, gay, bisexual, and transgender people may all identify with the word queer. Queer is sometimes used to express that sexuality and gender can be complicated, change over time, and might not fit neatly into either/or identities, like male or female, gay or straight.
The word “queer” has history to it that’s hurtful — “queer” used to be (and sometimes still is) used to put down or disrespect LGBT people. But more and more, people use the word with pride to identify themselves. So don’t call someone “queer” unless you know they’re cool with it. The best thing to do is ask what labels people prefer.