A tidy desk is a sign of a clear mind.

Meaning: This means that if your desk is neat and orderly, it can be a sign that your mind is also clear and organized.

Choice of Words: Tidy means neat and orderly. Sign means something that indicates or suggests something else. When you combine them, "A tidy desk is a sign of a clear mind" means that a neat and orderly desk can be a sign of a clear and organized mind.