For each report that is created you are able to add one or more filters. When filters have been added you are able to specify how to combine the filters - either AND or OR. By default the filters are evaluated as follows:
Where there are multiple ANDs and ORs then the expression is evaluated with AND given precedence.
If may be that you want to override this default order of evaluation. For example, You may have the following filter scenario: You want a report that shows all users are seated OR where the end date is set but that date has not yet passed. The filters would be
 User is Seated
 End-date is NULL
 End-date is after TODAY
By default the system would evaluate as (1 AND 2) OR 3 - because ANDs are evaluated first. But the logic requires 1 AND (2 OR 3). To force the system to evaluate in this later order you would need to alter the evalution order manually. First click on the evaluation order cog
You are then able to edit the expression (by adding brackets) to force the evaluation order required. In this example, we need to insert brackets to force the OR to be evaluated first