On the Dashboard, Orders Reports and a few other pages, comparisons to previous periods are used.
So for example if the current data range is set to the previous 7 days i.e. January 15th to January 21st.
Then the data from this range will be compared with the data from the previous 7 days which are between January 8th - January 14th.
You will then be able to see a percentage increase/decrease of sales etc.