The modern definition of the TOS byte is a six-bit Differentiated Services Code Point and a two-bit Explicit Congestion Notification field.
You can find each of these briefly defined on wikipedia.
In simple terms this field is now used in QoS (Quality of Service) to identify voice, video, data, routing protocols, etc. (voice, mission-critical, best-effort).
This is done to allow for providing priorities to each and then allocate bandwidth such that in times of congestion you still get an expected "Quality of Service"
This subject is covered in detail in the new CCNP:ONT class and certification guide.
