In this context they are probably referring to synchronizing multiple multitrack recorders together (often reel-to-reel digital machines). One would be the master and the other(s) slaves. This is done to increase track count, and also so that backup copies of the master tapes can be made easily and with matching time code. In addition, to protect the masters, a slave tape can be generated with a reference track of the master tape so that overdubs can be done on the slave tape. This tape can then be synched to the master and now the slave with the overdubs can be played with the basic tracks on the master tape. These days with MDM's, every other ADAT/DA78/ etc. is a slave to the master deck. The principles and applications are the same. BTW, it is possible to sync DAT machines, but they need to be pro machines with time code capability. I hope that this helps.