Filters allow you to sort your inbox by: Unread emails, Flagged emails, and emails with Attachments.
To use them, simply tap the Filters icon in the top right of the app to reveal a small menu with three sorting options: Unread, Flagged, and Attachments.
To remove the filter, tap the X icon to the right of the filter.
Note
Please note that Flagged is the same as Starred (for Gmail users).