From the paper that estimates the mentioned number:
"In the following we define the turnaround time of an accelerator storage ring as the time between the end of one and the start of the next physics run. For an accelerator storage ring the measurement of the turnaround time starts with the beam at top energy of the accelerator and comprises the ramp down of the magnet system to the injection energy settings after the beam
extraction, the time required for setting up the machine for the next injection, the time required for injecting new beams into the machine, the time required for the beam acceleration (ramp), the optics transition for the physics run (squeeze) and the time required for adjusting the beam conditions so that the detectors can start again data taking."
Essentially, it's the amount of time you need to stop the beam and start it again. The reason this is necessary is because, as particles collide, the intensity of the beam starts decreasing; at some point, the intensity (and therefore the collision rate) drops to the point where it would be more efficient to just stop, dump the rest, and refill the beam, rather than continue on collecting data at a lower rate. The precise point at which this happens is determined by how much time you lose when you have to stop and restart (i.e. the turn-around time).