As you go through your day you are bombarded with stimuli, many of which can effect the outlook with which you look at your day. Good news and progress in your goals can increase your outlook on life and add a little pep in your step, while hearing something bad or falling away from your objective can lead to dejection.
What happens when you have a mood disorder is that stimuli and the circumstances around you fail to influence your mood. Frequently the mind will ignore positive stimuli and continue on with a bout of depression, resulting in a loss of productivity and social drive. Conversely the mind may choose to ignore negative events in your life and carry on your happy demeanor, potentially offsetting friends and relatives when you fail to respond to tragedy in an appropriate manner.
A few examples of mental health issues that are classified as mood disorders are Bipolar disorder, in which alternating between extremes of various moods may occur, Major Depressive disorder where you experience pervasive sadness, and any number of other issues which can all impact your day to day life.