Complex sentence: The linemen and linebackers, close to the line of scrimmage, are often referred to as playing "in the box", players outside "the box" (usually cornerbacks and safeties) are collectively referred to as the "secondary".
Split into simpler sentences: The linemen and linebackers close to the line of scrimmage, are often referred to as playing "in the box". Players outside "the box" (usually cornerbacks and safeties) are collectively referred to as the "secondary".