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. In between those periods, they usually feel normal. You can think of the highs and the lows as two "poles" of mood, which is why it's called "bipolar" disorder.