Homophobia is the fear, hatred, mistrust of, or discomfort with, LGBTI people.
It is expressed through negative comments, rejection from friends and family, bullying, violence, discrimination, and negative media representation. Some LGBTI people can internalise homophobic attitudes, which is called self-stigma.
It is not only individuals who can be homophobic - governments and institutions can be too. For example, laws that criminalise homosexuality are homophobic laws.