What is the filter for?
With the filter, you can select a subset of all work units for the purpose of viewing them in the analytics section or exporting them to an Excel sheet.
How do I use the filter?
On Android, clicking on the filter button shows a dialog in which you can select the tasks and tags that should be considered. On Windows, the filter button is located in the "time tracking" page. Using it will show additional controls that you can use to filter work units by tag, task or search term. Both the tags and tasks filter you can invert to exclude tags or tasks from the result instead of including them. For tags, you can even choose from an AND-operator (show work units that contain tags A and B) or OR-operator (show work units that contain tags A or B).