Hello,
Shift is the 14hr period a driver is allowed to work after taking 10 consecutive hours off.
14-hour period. A driver may drive only during a period of 14 consecutive hours after coming on duty following 10 consecutive hours off duty. The driver may not drive after the end of the 14-consecutive-hour period without first taking 10 consecutive hours off duty.
https://www.fmcsa.dot.gov/regulations/title49/section/395.3
Hope this information helps. Please let me know if you have any other questions.