Strait, Channel
Noun
せと
今日市場ですごくいい瀬戸物を見つけたんだよ。
I found really nice chinaware at the market today.
The rapids door is like a door of water coming in to somewhere. This is a strait or channel. Also, this is the Seto area.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.