Non binary means that a person has a gender identity that is not male or female.
Non binary is the umbrella term for people that have an identity of not male or female although it can also refer to someone who experiences both male genders and female genders.
Or even people who don't identify with either gender.
The idea of there only being two genders is called the gender binary.
Non binary people may identify as genderfluid, genderqueer, bigender and agender.
Non binary people can also have any sexual orientation like straight, bisexual, gay or asexual.
And some people who are non binary also use pronouns that are gender neutral like them or they.
Some non binary celebrities are Emma D'Acry, Janelle Monae, Sam Smith, Demi Lovato.
Basically when a person says that they are non binary it means that the persons gender identity doesn't fall exclusively within the categories of female or male.
Essentially the person who is non binary does not identify as being soley a man or soley a woman and the non binary persons gender experience can include elements of both neither or something entirely different.