A front-end filter is a quick way to filter the dimension values in your report without having to edit them in the report builder.
You can apply a front end filter by clicking in the filter option in the report cog (found in the top right of the report).
This will open up the filter widget for the dimension. Select the type of filtering you would like to apply to that dimension then select the values you would like to filter.
To find out more on filtering types click here.