Agender (literally "without gender") is a term for people who do not identify with any gender, or whose identity does not include gender. The label is not a gender, in itself, but a declaration of the lack of gender.
The agender flag was created in 2014 by Salem X, after seeing a large influx of discussion around identities. Hearing the term agender stuck with them, so they decided to create a flag to represent it.
The black, gray, and white stripes represent a spectrum of identities, ranging from those who feel strongly about gender to those who don't feel it at all. The green stripe is the opposite of the purple stripes seen in the Genderqueer and Non-binary flags, which represent a mix of the traditional gendered colors.