Non-binary is an umbrella term that stretches out to cover basically any gender that isn't a binary man (he/him) or binary woman (she/her)

Non-binary people could be repulsed by the idea of gender or apathetic to the application of gender or frequently shift between genders, or represent two or more genders at the same time, or lean towards one gender but feel like they don't fall into that category, etc. etc.

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I'm not non-binary, but I have a feeling that people who use she/they or he/they are often non-binary enough to not consider themselves cis gender but really don't mind their assigned birth gender's pronouns out of familiarity or convenience. I'm sure there are others are who do feel like [binary pronoun]/they does fit them best though.