An Alternate Email is any email address other than your VO email address (e.g. email@example.com). It can be your secondary or personal email address, such as <name>@gmail.com, <name>@live.com, etc.
You can add an alternate email address to your VO account and use them to recover your account password.
If you happen to forget your VO account password or your account becomes compromised, you can request for a password reset and the confirmation link will be sent to the alternate email. (How to reset account password?)
To define your alternate email address, login to your VO account, go to Email → General.
Key in the email address on the given field under Alternate email.
Click Update to save your setting.