1to lie down or throw oneself down (on the floor, bed, etc.); similar to 寝転ぶ but often emphasizes lying fully stretched or reclining.
ソファに寝転がってテレビを見た。
I lay back on the sofa and watched TV.
公園の芝生に寝転がって空を眺めた。
I lay down on the park grass and looked up at the sky.
子どもが疲れて床に寝転がってしまった。
The child got tired and flopped down on the floor.