The flat or the flats (there is one on either side of the field) is the shallow area of the field beyond the line of scrimmage.
Sometimes it is defined as being outside of the hashmarks, but it is more properly identified as being outside of the normal defensive formation. On the screenshot above the flat is closer than the hashmarks at the top of the frame but further out on the bottom of the frame.
Gridiron Glossary is a resource for football terms that are often used by commentators, coaches and players but rarely defined. If there is a term you have questions about or a definition you don’t agree with, let me know at firstname.lastname@example.org.