This is related to being transgender. It is when people sometimes feel as if they're male and sometimes as if they're female. This does not have to be related to sexuality or physical gender in any way, and is not a result of indecision.
Kerry is physically female. Sometimes she feels male and wants to be called Keiran, and sometimes she feels male and is happy being a Kerry. So Kerry is genderfluid.
Gerald is physically male. Sometimes he feels female and wants to be called Geraldine, and sometimes he feels male and is happy being a Gerald. So Gerald is also genderfluid.