Open a browser window and enter the URL provided by welcome. In most cases, Advanced is the default client and is opened automatically when you log in.
Before you enter your name and password, you can switch to the standard web client or to the mobile client.
If you check this, you will not have to log in every time you restart the browser during a day. When this is checked and you do not log out, your session remains active until the session expires. The system administrator configures how long a session is active.
Note: If you leave your web client open in your browser but do not access your mailbox for a period of time, your session may time out and then you will need to log back in.
To prevent others from logging in to your email account, you should always use the Log Out link to close your session.
Source Zimbra website