Although, I had to look at an artists rendition as I don't think anyone has a close up picture of the flag.
![[User Posted Image]](https://pre00.deviantart.net/f3e5/th/pre/i/2012/094/5/7/avalar__s_flag_by_drakiusxsenna4ever-d4v1f92.png)
(credit to Darkius3000 on deviantart for this image)
So we have the stream, which represents Summer Forest. The golden bar, or "plain" if you will for Autumn Plains. And the Mountain is obviously Winter Tundra.
I think the blue orb at the top is supposed to be the sun. Or it could be that all 3 homeworlds are united under a single ruler and such they treat their ruler as if he or she is the sun, thus being at the very top.