Some people have a gender identity that differs from the usual man/woman binary. Those people are non-binary, because they don’t exclusively identify to one of those two categories. Non-binary people can identify on the spectrum of the man/woman binary, or completely outside of that spectrum.
Some non-binary people refute the idea that their genitals and their phenotype define their gender identity. They’re also against the western culture’s widespread notion that people’s gender is determined by the composition of their body (chromosomes, genitals, reproductive system, hormones, etc).