The term “homophobia” refers to when people are afraid of homosexuality. They are afraid of people who love people of the same gender: Lesbians and gays. In psychiatry, “phobia” refers to an anxiety disorder, that is, an anxiety that is irrational and inappropriate, that has no real cause. Homophobia creates prejudice against homosexual people and extends to hatred, aggression and violence.
Homophobia is very common in our society. There are lots of homo-jokes, i.e. jokes about lesbians and gays. On the other hand, there are almost no jokes about heterosexuality. The homo-jokes are often full of clichés and stereotypes, which means that they assign certain characteristics to lesbians or gays that are ridiculed. In contrast, many heterosexual people do not see themselves as heterosexual. For them, being heterosexual is not a characteristic that they have and that makes them a certain type of person. Being heterosexual is simply taken for granted in our society. Heterosexuality is just as much a sexual orientation, i.e. a characteristic, as being lesbian or gay.
Words like "queer" (you can find more on this under the keyword "hetero") or "gay" were once used as swear words. Those affected have nowadays appropriated and reinterpreted them, so that they have lost their negative meaning and are used quite normally. Although people who are not heterosexual are confident about their sexual orientation, the old prejudices still seem to persist in society. "Gay" is still partially used as an insult, often as early as Kindergarten age. On the other hand, no child would yell “You straight!” to another child!
As heterosexuality is apparently so natural and perceived as a matter of course in our society, and because it corresponds to society's expectations, it is also referred to as "forced heterosexuality" or "heterosexism". Heterosexism considers all people who are not heterosexual to be “abnormal”.