Genderfluid is a name for a gender identity that, simply speaking, moves between multiple genders. This can be any number of any genders, and can happen very quickly, very slowly, or anywhere in between.
Genderfluid people can have multiple presentations of themselves, each with a name, pronoun, appearance, and personality, or some, any, or none of those. Conversely, some genderfluid people present the same way no matter how they feel, keeping the same name, pronouns, appearance, etc. all the time.
Genderfluid people can use any pronouns, either changing or always the same. (A couple examples are they/them and ze/hir. Genderfluid people can use binary pronouns too. You can read all about it here.)
If you’ve figured out that it can be quite different for each genderfluid person, you’re right. Just like binary genders and other non-binary genders, being genderfluid feels different for everyone. As a result, people act differently. (That’s how nearly every aspect of a human being works.)
If you want to read more, search online for what genderfluid means, and read actual genderfluid people defining it. The best way to understand what genderfluidity is is to hear what genderfluid people have to say.