A filter represents a mean of restricting/grouping the data that are included in a reporting object. When you define a filter, you specify characteristics that the data must have to be included or filtered by. Once you are on Filters tab under Reporting objects you can define which questions (and newly created variables) will be available as filters throughout your reports. Simply select the relevant variables by ticking them in the 1st column on the left and hit save.
Filters can be set per chart, or per report, more information about setting this up can be found here.
Next step: create your portal tabs.