A timecode break is where your timecode on your digital video tape breaks from its direct line. Timecode acts as an address for events on a tape, so it is important to keep it in tact for editing. Timecode breaks can end up occurring for a number of different reasons. If you start and stop the tape quite a bit you will usually not break the timecode, though you may get some dropped frames. Usually you break the timecode if you record something, then rewind to watch it, then go back to the estimated location you were at.
The likelihood is that you will not go back to the exact frame where the video last left off, hence breaking the flow of the timecode. You can also break the timecode on your digital video tape if you have set your timecode to the time of day. This will actually break the timecode every time you turn the record off because when you return it will not be in exact sync.