Boat socks are very short socks, so short that you can't see the socks (or you can only see the edges of the socks) after wearing shoes, and the mouth of the socks is below the ankle.
The name "boat socks" is also called "ankle socks", "boat socks" and "super socks" because the shape of socks is like a boat and the length is up to the ankle. In Taiwan Province Province, most of them are called "invisible socks".