One should make a distinction between filters and filtering options.
The relationship is that of inclusion: filters consist of filtering options.
To enable a filter you need to select one or more filtering options that come with as a set of checkboxes.
Sometimes there are two or more filtering option subsets, e. g. the Priority filter has two subsets of options: Project priorities and Task priorities.
Every active filter you enable is displayed as a box directly above the filtered list of projects and activities.
The main elements of a filter include the icon that identifies the kind of filter, e. g. star corresponds to priority, dollar sign to money, etc. A filter consisting of two or more filtering options has a small blue circle on its top left corner, with the number of options at its center. It is demonstrated on the picture below. The filter shown on the picture is easy to interpret: the star tells you that it is a Priority filter with one of the filtering options being Medium and the other which is not shown but can be accessed by clicking the downward arrow at its right side.