Takuan is simply daikon radish, pickled.
Takuan is one of the most popular tsukemono (Japanese pickled vegetables). It is bright yellow in color, mildly sweet, tangy and very crunchy. Most often takuan is eaten as a snack or as one of many side dishes eaten alongside a main meal. It is also found in sushi rolls like oshinko-maki which is sushi rice rolled with takuan and wrapped with seaweed. In Korean, it’s called danmuji and is a necessary ingredient for making kimbap (Korean sushi rolls).
The first step of making takuan isn’t actually pickling but drying — daikon are first hung on special racks outside and sundried for a few weeks. After that, they’re pickled in a crock with salt and other ingredients in a process similar to that of making kimchi or sauerkraut. To give the radish it’s distinct yellow coloring, turmeric can be used, but in today’s modern time, food coloring is often added instead
Takuan can be bought whole or sliced, and you can find both of these in Uwajimaya’s refrigerated sections.