Actual genderfluid here, I can explain what this means
The simplest explanation that I can give is that the gender of a genderfluid person can and will change regularly and at inconsistent intervals. Sometimes it might happen in minutes, other times it can taken days, weeks, and in some cases, months.
Depending on the person, they might ask you to use different pronouns on different days, but others, like myself, will just use they/them to keep things simpler.
If you need anything else explained, just ask