A filter is simply a subgroup of your overall audience. To define a subgroup, you ask a question of the audience that will break them down into the categories you desire. That "Type of Question" in properties must be set to Filter Question. You can have many different filter questions.
For example, their specialty, position or title, how long they have been with the organization, male/female, there are a wide variety of ways to think about subgroups of an audience. You just have to determine what is important to you in reporting and/or what subgroup data that might be displayed, to help promote discussion within the group.