Non-binary describes a person whose gender identity is not exclusively man or woman, falling outside the traditional two-category system.
Non-binary describes people whose gender does not fit neatly into the categories of man or woman. It is an umbrella term covering many ways of experiencing gender outside that binary.
Some non-binary people feel like a blend of genders, some feel like neither, and some move between them. Genderfluid and agender identities often fall under this umbrella.
Many non-binary people use they and them as pronouns, though others use he, she, neopronouns, or a combination. The reliable approach is to ask and then use what a person tells you.
Gender identity is separate from sexual orientation. Knowing someone is non-binary tells you about their gender, not about who they are attracted to.
In dating, someone may share that they are non-binary so partners understand their identity and use the right language. Respect mostly comes down to listening and following their lead.
The term has become more visible as more people find that the two box model never described their experience.