Kaidō is a Japanese word that means "sea route" or "ocean road." It refers to a very important and very old series of trade routes that connect different regions of Japan by sea.
Think of it like a big map of roads that go across the ocean instead of on land. Just like how we have highways and roads to get from one city to another, these sea routes were used by people a long time ago to transport goods like rice, tea, and cloth from one part of Japan to another.
Nowadays, the kaido routes are still important for shipping and transportation, but they are also used by tourists who want to explore Japan's beautiful coastlines and small towns. So, even though kaido has a big history, it's still relevant today and is a really cool part of Japanese culture.