simple and clear; plain and simple
Describes an explanation, argument, or situation that is straightforward and easy to understand, without unnecessary complexity.
His explanation was simple and clear, and anyone could understand it.
この問題の解決策は単純明快だ。
The solution to this problem is plain and simple.
明快 means 'clear' or 'lucid' on its own, while 単純明快 emphasizes both simplicity and clarity together.
単純 means 'simple' or 'plain', but 単純明快 adds the nuance of being clearly understandable, not just uncomplicated.
A yojijukugo combining 単純 (simple) and 明快 (clear, lucid). The exact origin is uncertain, but it likely arose as a natural compound to emphasize straightforward clarity.