Mania is when a person with Bipolar goes from being very happy and friendly to being angry, or very, very sad.
"Mania" means that a person has a higher energy level in their brain. It does not mean that they go from being happy and friendly to being angry. A high-energy level could mean very, very happy which is called euphoric, or high-enegry could mean having high anxiety.
It is very important to realize that nobody in the world has a perfect brain. It is important to understand that everyone has problems. The study of psychology, is divided into two subjects: "normal psychology" and "abnormal psychology", yet since all people have imperfect brains, everybody has abnormal psychology in different ways because none of us are perfect and we are all unique.