poles apart; worlds apart; vast difference
A yojijukugo emphasizing an extreme difference, like that between clouds and mud, or a distance of ten thousand miles. Used in formal or literary contexts.
There is a world of difference between his way of thinking and mine.
Their abilities are poles apart.
雲泥の差 is a more common expression for a vast difference, while 雲泥万里 is a rarer, more emphatic yojijukugo.
A yojijukugo combining 雲泥 (clouds and mud, representing extreme difference) and 万里 (ten thousand miles, representing vast distance). The exact historical origin is uncertain, but it follows a common pattern of four-character idioms emphasizing contrast.