It refers to people who are one of the binary genders (man or woman) who are only attracted to people of a binary gender. They could be straight, gay, lesbian, bisexual, or otherwise multisexual so long as they are only attracted to people who are not non-binary. This includes cisgender and transgender people, so long as they fit in the gender binary.
The term was coined by Tumblr user acro-pluris on or before July 2nd, 2020.
The binian flag was made by Tumblr user Beyond MOGAI Pride Flag on July 2nd, 2020. It has no confirmed meaning.