In cases where you need a set of common filters applied on all the widgets in a tab, tab-level filters come handy. This is most often used when you want the tab to contain specific ad channels or products/brands while the report contains all of them.
This option exists just besides the New Widget option. Choose the dimension or metric with which you want to apply the filter, choose the relevant operator, enter the value(s) and click on "Apply Filter" to add the chosen filter in all the widgets of the tab.
Filters applied on tab and widgets would work in conjunction to each other.
Currently, tab filters support only 'AND' conditions. Note that filters added in this method will be editable only at the tab-level, and not on individual widgets.
Learn more about filters at the widget level.