straw mat
筵 centers on a woven straw mat used for sitting or as a floor covering. The meaning is concrete and specific to this type of traditional mat.
筵 combines 竹 (bamboo) with 延, which likely contributes the sound and the idea of spreading or extending, as a mat is spread out. The exact historical development of the component relationship is uncertain.
The top 竹 suggests bamboo or plant material, and the bottom 延 looks like something stretched out. Picture a bamboo-fiber mat spread across the floor: 筵 is a straw mat.
For エン, imagine the mat being unrolled at the end of a long hallway: end -> エン, and the mat stretches out smoothly.