If someone feels like a girl some days, a boy some days, and neither or both some days, and they're fluid between these three identities, then those identities can be combined into one, called trigender.
Here's a few examples:
"Today I feel kind of girly, but yesterday I was a complete and utter boy. Some days, though, I'm neither. I always rotate between those three identities."
"I don't feel like a boy or girl right now, but last week, I felt mostly like a combination of both. And yesterday, I only felt like a boy. I'm usually one of those three."
I hope that helps you understand the term better :)
There is also a thing called bigender, which is very similar to trigender. Except obviously, trigender is three, bigender is two. Here are some examples:
"Usually I feel like a complete boy, but there are some days when I feel like neither. I'm never a complete girl."
"Sometimes I feel like both a girl and a boy at the same time, and sometimes I feel like just a girl."
So that's my entry on those two identities. Thanks for reading :) next entry is about agender.