Let's consider the fictional Ada Byron, whose netid is adabyron and whose account is on tigger. The lock is to keep the inbox from being changed while the POP server is working with it. Then the POP server removes the file lock, opening Ada's inbox for new incoming email messages. The POP server then locks the . adabyron .pop file, and looks at it to see which messages it should download and which ones it's already seen. It keeps a lock on the file during the download.
Problems occur if the download is interrupted for whatever reason.
First, it leaves that the file being worked on locked, as the process that locked it hasn't been terminated. But not always. You can fix this and other problems from the ACCC Email Tools page.