Kobukusa is about one-fourth the size of a Fukusa. Other items are placed on top of it (in the lower level temaes), either to show that one has a high regard for that particular item (Kazari temae), or to protect the item from the ground (Chabako).
Most guests will have two of these with them when attending a tea gathering.
Other schools use a Dashi-bukusa 出帛紗 instead of a kobukusa. In Urasenke a Dashi-bukusa may always replace a kobukusa and is used the same way. A slightly smaller kobukusa called Shiki-kobukusa 敷古帛紗 is used in Chitose Bon.