The top bar shows your performance today. The bottom bar shows your performance on mature cards over the life of the deck. The bold text in the tooltip corresponds to the bars.
Mature cards are cards that have an interval over 21 days. When you start a new deck, you won't have any mature cards, so the bottom bar will not be shown for the first month or so.
The bottom bar should generally be between 80-95% - lower and you're forgetting mature cards too often, higher and you're spending too much time reviewing.