Sometimes when you are running a Manger module option it is important to ensure that nobody logs into Herschel. The lock / unlock system option allows you to prevent users logging in and then to unlock the system again. When you lock the system anyone using Herschel will be warned the system is closing, and asked to log out. If they don't log out, they will be forced out after five minutes:

If you accidentally lock yourself out of the system then you can gain access again by deleting the file LOCK.TXT in your \HERSCHEL folder.