Complex sentence: Time-sharing means that many persons can access MVS concurrently but each is unaware that the operating system is being accessed by others-so it appears to each TSO user that he or she is the only user on the system.
Split into simpler sentences: Time-sharing means that many persons can access the operating system concurrently, while unaware that others are also accessing the operating system. It appears to each TSO user that they are the only user on the system.