light意思是：轻，也就是份量很轻的轻；feather就是羽毛，羽毛的份量确实是很轻的。as light as a feather那末顾名思义就是：像羽毛那样轻；如果是形容食品，意思是：清淡的。

【例句】
The little baby rabbit was as light as a feather in her hand.
这只小小的幼兔在她手里像羽毛一样轻。

The white chocolate cake is not that sweet and is as light as a feather.
那个白巧克力蛋糕不是太甜，非常清淡。