Complex sentence: The defensive backfield (also known as the secondary) consists of cornerbacks (CB) and safeties (S); safeties are themselves divided into free safeties (FS) and strong safeties (SS).
Split into simpler sentences: The defensive backfield, often called the "secondary," consists of cornerbacks (CB) and safeties (S). Safeties are themselves divided into free safeties (FS) and strong safeties (SS).