I'm no expert, but with bipolar, you tend to experience 'highs', defined as being periods of mania, as well as lows. During the 'highs' you might have a lot of energy, overly ambitious plans, feel unusually happy and maybe not feel so much need to eat or sleep - like you're just kind of functioning on another level. Then you also have 'lows' which is like your normal depression, so feeling tired, worthless, guilt, despair, no energy and all that sort of stuff.
You might have phases of highs more often than lows, or the other way around. And the severity also can change from person to person, and also within a person during the course of their lifetime. You might cycle rapidly between highs and lows, or have long periods of lows with the occasional extreme high, or you might just have it all kind of mixed together, so like overactivity and ambitions with a depressed mood.
If you have any more questions feel free to tag me.