to lie down; to throw oneself down
疲れてソファに寝転んだ。
I was tired and lay down on the sofa.
子供が芝生の上に寝転んで空を眺めている。
The children are lying on the grass and gazing at the sky.
休日は何もせずに寝転ぶのが一番の贅沢だと思っている。
I think lying around doing nothing on days off is the ultimate luxury.