From the php man page on Session usage
Sessions follow a simple workflow. When a session is started, PHP will either retrieve an existing session using the ID passed (usually from a session cookie) or if no session is passed it will create a new session. PHP will populate the $_SESSION superglobal with any session data after the session has started. When PHP shuts down, it will automatically take the contents of the $_SESSION superglobal, serialize it, and send it for storage using the session save handler.
So, to answer your question, it means there's one read at session start, and one write at session shutdown, in between, the $_SESSION
superglobal is essentially held in memory.