Filters has been built to help you focus on the specific piece of data that's relevant to your analysis at any given time, or to hide low-interest data. It does this by allowing you to set conditions on both dimensions and metrics, and only showing the data that matches those conditions.
Please note that if you have editing access of a report, all filters that you apply to a tab/widget are saved automatically and remain so until you/another editor removes them. If you only have viewing access, you can still apply filters for exploring data on-the-fly but they will not be saved once you exit the report.
Let's illustrate with an example, how a filter works.
The following sample widget shows Spend, Impressions, Clicks, CPC, CTR and Revenue for Facebook and AdWords campaigns. Consider you want to look only at the brand campaigns in AdWords and retargeting campaigns in Facebook. To apply a filter, you need to -
Click on the "Filter" button in the widget navigation bar, select the dimension or metric that you wish to apply the filter to.
Select the operator which you wish to apply to the chosen dimension/metric.
Enter the value to filter by and click on "Apply Filter".
To illustrate the specific case explained above -
The filter will be applied instantaneously, and only the filtered data will be visible. Please note that the aggregate shown will also change to reflect the filters.
You can also add filters to metrics, using operators such as "equals", "not equals" "greater than" and "less than" by following the same procedure. For dimensions, the supported operators are "is", "is not", "contains", "does not contain" and "starts with".
You can combine multiple filters with 'AND' inside a block, and have multiple blocks with 'OR' between them, as illustrated above. Also, any filters applied at tab-level will be applied on the data before the widget-level ones are.