(Why yes, I am using this because I was looking into 焉 for the previous kotowaza post. Funny how that works.)
Literally: bird – how / why – foolish – fish
Alternately: The meanings of the original characters don’t actually matter here! Instead, the point is that 烏 and 焉 are similar, as are 魯 and 魚. Put together, this pair of pairs refers to the act of mis-writing; of accidentally replacing one character with a similar but incorrect one. The kanji equivalent of a homophone-based spelling error.
Notes: There are a number of variations on this theme, such as 魯魚亥豕 (rogyo gaishi), which moves the fishalikes to the front and replaces the birdalikes with 亥 (“boar”) and 豕 (“pig”).
One bit of kanji trivia is that 鳥 can also mean “crow” rather than the more generic “bird,” while small birds have been more specifically represented by 隹. In modern Japanese, though, the former character has been generalized, while the latter almost exclusively appears as a single element of more complex characters. (Examples include 隼 = hayabusa, “falcon,” 雀 = suzume, “sparrow,” 鶴 = tsuru, “crane,” and so on.)