It is a disease that is thought to be caused by a chemical imbalance in the brain. It can manifest itself by manic and depressive 'mood' swings. It needs to be diagnosed by a doctor. Oftentimes medicine can help the person manage these 'swings'.
It means you have very high highs & very low lows & no in-between, it's caused by a chemical in balance in the brain & can be helped by certain drugs. People with bi-polar tend to get manic highs where they spend a lot of money, love everybody & everything & can't settle down. Very low lows where they don't get out of bed & may try to commit suicide. It's not where their mood switches every 2 seconds!!