Google explains it well, to be honest:
Bipolar disorder, also known as manic depression, is a mental illness that brings severe high and low moods and changes in sleep, energy, thinking, and behavior. People who have bipolar disorder can have periods in which they feel overly happy and energized and other periods of feeling very sad, hopeless, and sluggish.
They can suddenly get extremely energetic, happy and whatnot to a degree that would seem "manic" or "crazy" to those on the outside then without warning have a period of being extremely low and depressive. Just a rollercoaster ride of emotions. It can manifest in other ways too. Sudden anger, aggression and so forth then suddenly seeming so remorseful when they finally wind down. I'm no expert at all but that's the gist from what I understand. It's quite a miserable thing to have to deal with and can be very, very exhausting and traumatizing for those around them too.