The Address Book allows you to store the email addresses for people you email frequently. When composing an email, you will be able to automatically populate the "To", "Cc", or "Bcc" fields from your stored Address Book contacts.
Please note that the contacts you store in Webmail will only be accessible within the Webmail interface. However, you can optionally import or export contacts in your Webmail Address Book to/from other popular email clients such as Microsoft Outlook or Mozilla Thunderbird.