As you have probably seen in the world around you, different people are attracted to different genders.
If someone is only attracted to the same gender, they can be described as homosexual. If someone is attracted only to a different gender to themselves they are heterosexual. In fact, the homo- part of the word means “the same” while hetero- means “different”.
You will sometimes find that people use “heterosexual” to mean something more narrow – attraction specifically between and man and a woman – or what used to be thought of as “opposite sexes”. It gets used in that way, but that doesn’t really fit with the idea of gender being more complex than just “men” and “women”.
Many people find that they are attracted to people of more than one gender, or that they don’t really mind what body a person has, it’s the person inside that they are attracted to. If so, that person may describe themselves as bisexual or pansexual.