Tregender (pronounced treh-gender) is a form of genderfluidity in which one fluctuates only or mainly between masculine, feminine, and female-related genders. For example, one may fluctuate between rosboy, demigirl, and female, but never binary male.
The male equivalent is draigender.
The term was coined by FANDOM user Hiddn.haydn on September 9 of 2021, while the flag was created and updated by the same user on September 13 of 2021. 
The flag is meant to represent the three main aspects of one's identity. Brown stands for masculine genders, dark pink stands for womanhood, and light pink stands for femininity. The in-between shades stand for fluctuation between identities.
The prefix "tre" comes from the Italian word for number three,  as romance languages are usually associated with femininity, especially in contrast to Slavic and Germanic languages.