It is the attraction you feel towards those of your own sex. Being gay is where two males (or two females) love each other in a sexual way. (Another term for gay females is
lesbian.)
Most gay people are born that way. They sooner or later realize that they are attracted to the same gender.
Some individuals consider themselves bisexual, and may have sexual relationships with both men and women.
What makes a homosexual person homosexual is genes. It is not something hereditary or learned, but a natural feeling that is discovered sometime during the teen years. The same goes for bisexual and heterosexual people; sexuality varies among people, and always will. It is natural that any given human being would be attracted to the same sex, to the opposite sex, or to both.
Homosexual behavior has been observed in most animals, including birds. The basic tendency towards homosexuality or heterosexuality is probably genetic, and appears to be fixed before the age of six. The ways in which sexual orientation are expressed are socially determined.
Gay is just because we were born that way. We did not wake up one day and decide it. It was just meant to be.