Flexisexual when your gender is fluid and your attraction changes with it or your gender changes with your crushes gender preference. You might be feeling more feminine and be attracted to a hetero man, or feeling more masculine and be attracted to a homo man. You might like someone, and feel yourself changing towards their gender preference.
Another definition could be that it stands for a sexuality in which one's sexual attraction changes over time, sometimes on a day to day basis, sometimes over the course of a month, depends on the person for the exact time. In labeling, one might just say they are flexisexual, or put another label for their main or most common attraction, or even put a label for the range their attraction goes through