Although zafu is often translated as "sewn seat" in American English, the meaning of the Japanese kanji, 座蒲, is different.
A zafu is a seat stuffed with the fluffy, soft, downy fibres of the disintegrating reedmace seed heads.
The cushion raises the hips, making the entire range of cross-legged sitting positions more stable for the meditator.
In many practice places, there is a prescribed form for respectfully handling zafu while walking in the meditation hall, or zendo.
The outer cover is typically made of a heavy duty fabric and has a zipper along one side so that it can be easily removed and washed.