What does food on best / worst days mean?

Your food on the best and worst days is calculated based on your symptom score. For the analysis to work, you must track your meals, including every single ingredient and symptoms for at least 6 days. It is also important to track the lack of any symptoms

The overview for the best days shows foods with a low symptom score and the overview with the worst foods shows foods that caused a high symptom score.  
Note here that any food you ate before the symptoms occurred will also be displayed. This means that the foods listed serve as an indication of triggering foods, but have to be checked individually. 

In case the same ingredients are displayed on your best / worst days, you can click on the small gear icon at the bottom right of the respective list and adjust the settings by choosing to hide duplicates.