最后一行说，x:xs的子集集是xs的子集集，而x都添加到了这些集中

The last line says that the set of subsets of x:xs is the set of subsets of xs along with x added to each of those sets.

另一种表达方式是

zs <- [ys,(x:ys)]

为每个xs的子集之一精确地生成两个集ys和x:ys.

generates exactly two sets, ys and x:ys for each ys which is one of the subsets of xs.