It stands for " rolling on the floor laughing."

它代表“笑得在地板上打滚” 。

And if you had of told me that I'd have a Youtube channel and I would produce videos that got sent out to the world every week, I would be rolling on the floor laughing.

如果你告诉我我会拥有自己的油管频道，我每周都会发视频，发布到世界各地，我肯定会在地板上哈哈大笑的。