interlocking can be considered in several ways.
stop and go signals between machines is a good example.
consider a large press that makes car doors and has 3 machines.
#1 robot to load the part.
#2 the press itself
#3 the robot to unload the part.
you press the start button (machine is empty).
the system looks to see of everything is in the home position.
yes, keep going - no give an error
1. robot 1 loads the part and goes home.
a part loaded signal is sent to the press.
2. the press operates and goes home.
a part processed signal is sent to robot #2
3. robot #2 removes the part and goes home.
a part removed signal is sent to robot #1 to load the next part (step 1)
this is a very simplified version of interlocks, but you can see that each machine depends on the signals from the others.
I hope this helps,
james