“Who can (make) the muddy water (clear)? Let it be still, and it will gradually become clear.”
The muddy-water image makes stillness a condition for clarity: agitation can keep disturbance suspended, while enough quiet allows what is clouding perception to settle without force.
Source details
Tao Teh King, Chapter 15, James Legge translation, Project Gutenberg #216. The next sentence asks how the condition of rest is secured and answers through the gradual relation between movement and rest; the passage belongs to Taoist philosophy, not a clinical relaxation technique.