It’s a grass used for flavour (and colour) which is slightly sweet/herbal. It makes for a really lovely vodka in my opinion, often you have it mixed with Apple Juice.

https://en.wikipedia.org/wiki/Żubrówka

https://en.wikipedia.org/wiki/Hierochloe_odorata

Żubrówka or Żubrówka Biała if you aren’t into the bison grass (the bison grass is pretty good, though). They’re made in Poland, and the parent companies are Polish.