Record enable & safe mode   

Multitrack recorders provide a switch for each track which allows the user to either enable or disable recording on that track. When a track's ability to record is disabled it is said to be in safe mode. 
Always be sure tracks are in safe mode unless you are recording on them! Otherwise you risk erasing prerecorded material.  Conversely, if you end up with no recording on a track after pressing the record button on the machine and sending signal to the appropriate channel it may be that you neglected to enable recording for that track. 
On many modular digital multitracks (ADAT, DA88) an input to track 1 is also sent to 3, 5 & 7 and an input to track 2 is also sent to 4, 6 & 8. Record enable switches also allow the user to direct the signal to the appropriate track.