As we all know that gender is a societally constructed phenomenon, we as a part of society have taken the basic guidelines of gender quite literally. From the toys we play with to the chores we do at home, everything is divided based on our gender. When it comes to deconstructing gender, while most believe that our Biological sex determines our gender, in reality, it's the amalgamation of three important components which are: Biological sex, Gender Identity, and Gender Expression. The combination of these three factors is what helps a person determine their 'gender'.
When a person says that they identify themselves as a Genderfluid person, all they mean to say is that while their biological sex remains stagnant, their Gender Identity and Gender Expression keep changing depending on the situation they are in.
There might be a day where they might feel masculine, while other days they might feel feminine and this is completely normal to happen. While we all may feel this from time to time, depending on how often they feel this way, people term themselves, Genderfluid people. While or before addressing such a person it is always advised to ask them about their preferred pronouns without assuming or stereotyping them.