Bisexuality
Bi-Sexuality
Bi- Latin multiple prefix denoting the value 2
Sexuality- The way someone is attracted to other people
2 - attraction to other people
Both - Sexualities
Thus - Attracted sexually to both genders.
It's really not difficult.
Also, remember that some people can also be pansexual (sexually attracted to people of all gender identities and biological sexes) have aesthetic sexuality (sexually aroused by beauty, including but not limited to paintings, beautiful sculptures, natural beauty)
Basically, while you may not be able to comprehend it, the same thoughts you may only get when seeing a beautiful woman (I'm assuming you're a heterosexual man), a bisexual person could feel for either a man or a woman, and a pansexual could feel for literally anyone. Or even Asexual (no sexual attraction, which doesn't rule out romantic attraction)
Not that 'could' does niot necessarily denote 'will.' It's a common misconception that bisexuals are somehow sexually predacious, but think rationally for a moment. Do you want to have sex with every single woman you see, and does every single woman want to have sex with every single man? No. Bisexuality doesn't make you promiscuous, it just gives you awider field of people who might make you horny.
Sexuality is complicated.