it means that when Emergency-stop is enabled and all the trains come to a stop, when you disable it, it needs to be able to get back to the station to pass.
I read somewhere that the E-Stop test involves hitting the F12 key (emergency stop) just after your first train finishes the lift hill. All trains should come to rest without any accidents
You can press e-Stop whenever you want (this is what we do in real life when needed).. but yes, all train must finish their ride to the next e-Stop without crash.. and when release E-Stop, continue like nothing happened..
This is why it's generally a good idea, in my opinion, to have too many brake blocks and either to slant them or (if needed) add friction wheels so everything will still make the circuit.