Non-binary means that you are not limited in the self (as opposed to the body) to the concepts of male and female. While masculine and feminine are social constructs, male and female are the objective items that they are applied to. However there is a problem that there is a disagreement as to what exactly are the objective criteria for male and female. Genitalia, chromosomes and genes don't necessarily match each other. With that said, and understanding that a change in concept and knowledge requires a shifting of language, sex (referring to the body) and gender (referring to the self) are becoming distinct words, no longer synonymous. Likewise, male and female are no longer synonymous with man and woman, respectively. This particular lingual shift, however, is meeting with more resistance than the previous. Such is the nature of language evolution. Keep in mind that non-binary is different and separate from transgender, which is where the sex is opposite the gender.
Most people know whether they are a man or woman, even as children, regardless of whatever gender expressions (also social constructs) they present. Thus one can present all of the gender expressions of women, have the genitalia of a female, and still know that they are a man. Now, statistically speaking, the vast majority of people have their sex and gender match. But some contain a mixture of both genders (as opposed to sex, which people can have a mixture of those traits as well), in varying degrees and not necessarily in a fixed ratio, or in some cases, neither. These are your non-binary people. For those whose point along the line shifts, they are typically labeled gender fluid, or gender flex. The later tend to stick to one point or the other, but not always, hence flex vs fluid. Some are neither. In all cases, they may utilize gender expressions from both of the current societal expectations. This includes people whose gender and sex match. As a man and a male, I can decide that skirts are more comfortable than slacks and decide to wear them instead. I'm not trans, or non-binary or anything like that. I am simply not following the social construct. That is how fashions changes across time. High heels were initially a male item. Every type of clothing out there (which the possible exception of skorts, which are relatively new) has been worn by both men and women at one point or another in history, even if they had other names to try to distinguish them from the rest. Face it, a kilt is a particular style of wrap around skirt.
Something else to keep in mind is that their gender, be it either pole or non-binary, has nothing to do with their sexual orientation. The labels for those are rather messed up especially once we account for the issues of gender vs sex. I find it better to think of attracted to men or women or males or females (the difference can be key in many cases). Usually a person who is attracted to women to start with, and then transitions into the opposite sex, is still attracted to women. Their orientation itself doesn't change. The label for it does, but not the attraction.
As far as your loved one goes, they may be truly non-binary. Or they may simply be bucking the current gender expressions, and being labeled as non-binary, even if not truly so, makes it easier to explain, or cope with or both.
For context, I am not non-binary myself, but I am part of the poly and kink communities and deal with a large (relatively speaking) number of transgender and non-binary people. The above is what I have learned from them, both in conversation and observation.