清-Sei is the third Chinese charactre of "wa-kei-sei-jaku."
和敬清寂の三番目の言葉は「清」です。
Sei means purity.
清らかということです。
The host purifies utensils with a silk cloth.
亭主は袱紗で道具を清めます。
The host purifies not only them but also one's heart.
道具のみならず心も清めます。
Everythig is pure and clean in a tea room.
茶室では全てが清らかです。