Meditation is the art of silencing the mind, allowing concentration to increase and inner peace to emerge in the midst of a world full of turmoil. To understand how this constant turmoil can negatively affect your mind, you need to look at your mind like a lake. When someone throws a rock into the lake, or a boat passes through it, ripples and waves are created, rising until they reach the shore and disappear. Now imagine your thoughts are the waves inside your mind.
Just like throwing something in the lake will create ripples and waves, being regularly exposed to the negative thoughts and overthinking what happened in the past or the future, will distort your mind's calmness and leave you feeling anxious, tense, worried and stressed.