What is meant by "overhead"

What is meant by "overhead" in this sentence " DS-1 requires 8 Kbps of overhead" ?

A DS1 circuit is made up of twenty-four 8-bit channels (also known as timeslots), each channel being a 64 kbit/s (multiplexed) carrier circuit. A DS1 is full-duplex circuit, which means the circuit transmits and receives 1.544 Mbit/s concurrently. A total of 1.536 Mbit/s of bandwidth is achieved by sampling each of the twenty-four 8-bits, 8000 times per second .With these an additional 8 kbit/s of overhead is also obtained from the placement of one framing bit that make total bandwidth of 1.544 Mbit/s.