What does the 14 hour on duty limit mean?

The 14 Hour Duty Limit
This limit is usually thought of as a "daily" limit, even though it is not based on a 24 hour period. You are allowed a period of 14 consecutive-hours of duty time after being off duty for 10 or more consecutive hours. The 14 consecutive hour duty period begins when you start any kind of work (performing vehicle maintenance, loading / unloading cargo, fueling, etc.). Once you have reached the end of this 14 consecutive hour period, you cannot drive again until you have been off duty for another 10 consecutive hours.
Your driving is limited to the 14 consecutive hour duty period even if you take some off duty time, such as a lunch break or a nap, during those 14 hours.
Example: You have had 10 continuous hours off and you begin working at 6:00 a.m. (vehicle inspection, fueling, loading / unloading freight, etc.). You must not drive your truck after 8:00 p.m. that evening. You may do other work after 8:00 p.m.(load / unload freight, perform maintenance duties, etc.), but you cannot do any more driving until you have taken another 10 consecutive hours off.