Your performance shows how much money you have gained by trading in various markets across the different time periods. For example, if you gained $10 today, your "today" total would be 10. 10 would also be added to "this week," "this month," and "this year's" totals.
Each time you gain or lose money, they are added or subtracted from the appropriate time periods.
If the totals don't seem right for the day, you may want to check again the following day. Performance amounts are only re-calculated once a day.