There's a Help topic for that: How do filters work?
The filters can be accessed by clicking on the "Show" button in the toolbar above your list. Certain filters may not be available to you if you are not using the related field. You can pick which fields you are using in your account settings. Here are the current filters:
Recently Completed
This filter will hide or reveal tasks that were completed in the last 7 days. Tasks that were completed more than 7 days ago are always hidden and can only be viewed by conducting a search.
BTW, all (or practically all) the features in the iPhone/iPad apps are also on the website, so if you see a feature on the iOS app, then it is almost certainly available on the website.
