Psychology is the science of mind and behavior. Psychology includes the study of conscious and unconscious phenomena, as well as feelings and thought. Also, understanding of the emergent properties of brains, linking the discipline to neuroscienceWikipedia
To know yourself and understand why you do the things you do. When you know your psychology your behavior becomes more predictable and less random. You understand why you and people do certain things. In its simplest form there are only a few things that motivate people. Once you know what the motivator is you can dangle it like a carrot to make someone take action.