The Rubaka is a shirt or dress that is worn by men or women, respectively. These shirts are the foundation layer for a Russian. They can be yoked or not and are typically long-sleeved and have gores on the sides. The neck opening is either in the middle of the front or just to the side as demonstrated here.
Faolan is modeling a man's rubaka, which is nicely piped in green. Tatjana wears a rubaka under her over-tunic or "naverschnik" (you can see the red sleeves under the blue naverschnik).
Women would wear a linen rubaka as an undergarment, then usually another rubaka of linen, silk, or wool over that. The outer rubaka was more ornate. Additional layers of garments would be added as the weather dictated..