NPR has put together a glossary of terms relating to gender identity who's goal is to help people communicate accurately and respectfully with one another.

What's a pronoun, why they matter, and what to do if you make a mistake.

Preferring they/them pronouns isn't synonymous with being nonbinary. After all, pronouns describe one's identity — they don't define it. Read here for "What People Get Wrong About They/Them Pronouns".