You'll see this available in little plastic bags on the street, all prepped and peeled like this. It is white turmeric, similar to ginger. It's eaten raw like this, with chilli pastes (Nam Prik) as a dipping sauce.
It has a medicinal edge to it, it reminds me of swimming pool chlorine. Often claimed to have health benefits (even cancer fighting benefits), but that's typical of anything that tastes bitter or medicinal.
When eaten with Nam-Prik it blends to a sort of minty taste. An acquired taste.