Filters can be used on a project's Activity Section, the Task List, or on the Feed. They let you restrict your view to items that match certain criteria while hiding any items that do not.
Activity Section Filters
On a project's Activity Section, click "Filter" in the top right, under the "write a note" box. From the list that appears, choose the items you'd like to be visible on your Activity Section.
Once a filter has been added, you can use the Filter menu to add additional criteria if needed. Selected filters will stack up alongside the Filter button, letting you know what filters are currently in place.
To clear a filter, click the "X" button by the filter name.
On your Feed, you can add filters by using the same Filter menu in the top right.
Task List Filters
On your Task List, you can add filters by using the same Filter menu in the top right. Since only tasks can appear on your Task List, you will not have as many options for criteria here as you did for the Activity Section and Feed.