In English one refers to a male as "him" and a female as "her". However, some people identify as "non-binary", which means that in their own internal self they so not feel that they are entirely male, or entirely female. As a result, they find it not quite right to be referred to as "him", but also not quite right to be referred to as "her".
So they prefer to be referred to in the gender-neutral "them".
If you are "traditionally aligned", if your personal self-identity matches the body your ghost[0] inhabits, this can be totally mystifying. Personally, I've come to accept that people are weird, and if someone says they want to be referred to as "they" and "them" then why not?
[0] "You are a ghost driving a meat covered skeleton made from stardust[1] riding a rock floating through space."
[1] Or nuclear waste - take your pick.
Edit: To the people downvoting this, it was intended to be a factual answer to a direct and reasonable question. I have no idea why you think this comment is inappropriate and deserves a downvote.