In this section you will see all fields with always the same value in the filtered view. These fields are automatically removed from the log rows grid and from the selected row panel as they don't change; there is no need to display them there.
This section will be mostly empty when no filter is applied but gradually as you filter down the view you will see more fields in here and less in the grid view.
Right to the field name the corresponding value is displayed and it can be clicked. If you do that the view will be filtered on this value and any other filter will be removed.
For example, let's suppose you gradually filter down the view with various conditions but not on the IP address (ClientIP) but at this step you see that only accesses from one IP address match the filter. You would then have the question: But was there any other activity from this IP address outside this filter?
To know that you just need to click on the IP address in the Constant field section and the filter of the view will be extended by showing all web requests from this IP address and you will get your answer.
There is also a ( - ) link at the right of the value. If you click on it and there is currently a filter on this value in the filter box, this filter will be removed and the view refreshed accordingly.