The Filter View gives you an alternate way to view a set of Tasks that all meet certain criteria. In the Filter View, you can choose to filter Tasks by their due dates or by Tags assigned.
The Filter View has built in “filters” that allow you to see Tasks due today, overdue Tasks, and upcoming Tasks. You can also choose a Tag as a filter to see the set of all Tasks that have that Tag assigned.
To access the Filter View on the desktop, click the button three vertical bars on the right side of the titlebar area. This button is used to toggle the Filter View on and off.
On iOS, the Filter View is the 3rd tab at the bottom of the app.
See: How do I use Tags?