The "Blank" and "Param" checkboxes (see image above) allow you to control the filtering behavior in the report viewer. They do not affect the “Preview” tab of the report designer.
Blank Checkbox: To use this, first set a “Filter Field” and then set an “Operator”. A value can be entered, but it is not required. If “Blank” is checked, the filter will return data that matches the “Value(s)” dropdown/textbox and also data that has a blank/null value in that field.
Param Checkbox: To use this, first set a “Filter Field”, set an “Operator”, and then set “Value(s). This will display the filtered report in the report viewer and allow the user to change the filter value. If it is not checked, then the filter will not be visible to the end-user and the end-user will not be able to change the filter.