Oki so heterosexual basically means you are straight (sexually attracted to people of the opposite sex). For example: If a boy falls in love with a girl and doesn't like boys or any other gender orientation he is generally straight.
BUT REMEMBER!! Just because someone is straight doesn't mean they are homophobic!! They can still support LGBTQ+ even if they aren't part of the lgbt community. The flag bellow is the straight allies flag which a straight person can show if they wish as a symbol of supporting.