This is the 7279th most frequent Japanese word.
中綿
Padding or filling (used in clothing or quilts).
Here '中綿' refers to the insulating material inside the jacket.
このジャケットの中綿は軽くて暖かいです。
This jacket's padding is lightweight and warm.
In this context, '中綿' signifies the filling inside the pillow.
手作りの枕は中綿が硬すぎないのが特徴です。
Handmade pillows are characterized by padding that isn't too firm.
The word '中綿' here describes the material added for warmth in the futon.