straw mat, matting
蓆 centers on a woven floor covering made from plant material: a straw mat or matting.
蓆 combines 艹 (grass) with 席 (seat/mat), forming a character for a grass mat. The historical development is straightforward, with 席 likely contributing both sound and meaning.
Grass 艹 on top of a seat 席 makes a straw mat: imagine a woven grass covering placed over a seating area.
For セキ, picture a straw mat placed on a stone seat: the stone seat cues セキ (石, せき) while the grass mat covers it.