Bisexuality is one of the more commonly misunderstood of all sexualities, next to transsexuality. Some of the most common misconceptions is that it is a passing phase, whether into homosexuality (being gay) or from that into heterosexuality (being straight). Most often bisexual people are considered confused, and wierd, and many misinformed people are convinced that one can only be gay or straight.
The attraction to 1) people with the same gender and 2) people of other genders. This is often confused with pansexuality, but bisexuals are often differentiated by things like having different tastes in one gender vs. others (e.g. being a woman who likes loud women, but quiet men) or not being attracted to all genders (e.g. being a man who likes people who are agender, bigender, and other men, but not being attracted to women).
Sometimes, I'll be walking and I'll notice a handsome man and then I'll see a gorgeous woman and I'll just smile to myself and be really glad for bisexuality.
When somebody is attracted to people of both sexes. Yes, some people claim to be bisexual for attention, but there are many people who are genuinely bi. I'm not sure why people don't think bisexuality exists when it clearly does (it's well-established).
Hattie's bisexuality made her less popular in her homophobic school but she still kept up her bisexuality because she was bisexual, not because she was attention-seeking.