Consider the analogy of a stagnant pool of water which has become murky and general filth from daily life, there is no fresh water entering the pool to cleanse and refresh it. You cannot even see your reflection in the water because all that can be seen is cloudy stagnant water with all kind of bugs in it.
Now visualise a beautiful pool with crystal-clear sparkling water with rays of sunlight and birds dipping in the water and you can see all the way to the bottom of the pool and the and when the water is still you can also see a mirror-perfect reflection of yourself.
Now think of your mind on a particularly bad day when your thoughts are in chaos, when you are feeling angry or another negative emotion that creates turmoil, it feels as though there is no way to ease the intensity of your thoughts and the seeming pressure inside your brain from the excessive analysing and strong emotional activity. This is the murky water scenario.