I understand what the activists mean by the terms “gay,” “lesbian,” “bisexual” and “transgender,” but what does the term “queer” mean that isn’t already covered by the other terms?
It's an umbrella term for homosexuals, those who a are neither heterosexual or cisgender (a person whose sense of personal identity and gender corresponds with their birth sex). Homosexuals do not have one set meaning for it so i suppose it is included just to cover all bases.