Danmuji (단무지) or takuan is pickled daikon radish. This is a key ingredient in kimbap (김밥). The tang of the vinegar gives it a bite and crunchy texture that is a important element to the dish. This can also be eaten as a side dish, especially with jajangmyeon (짜장면), Korean Chinese-style noodles with black bean sauce.
Danmuji can be found in any Korean or Japanese market. They are generally sold in packages of one to four long tubular shapes of bright yellow (dyed) or white. You can also buy them precut in long strips for sushi/kimbap.
Store in the fridge and it stays good for a long time as with most pickled items.
For kimbap, cut into long cubular strips.
For a side dish, cut the tube in half lengthwise and then cut into thin half moon shapes.
Read here for making your own danmuji, as well as, methods for naturally dying it.
Find a Korean market in the Boston area.