An email "alias" is a pseudo email account, where all email sent to the alias gets auto-forwarded to another email address. Example:

Let's say that you have an email account named "webmaster".  This would make your email address "webmaster@yourdomain.com". Next, if we create an alias and call the alias "info", we can then point the alias to the "webmaster" account. What this does is auto-forward all email sent to info@yourdomain.com to webmaster@yourdomain.com 's email account. When user "webmaster" fetches his email, he will be retrieving email for both "info@yourdomain.com" and "webmaster@yourdomain.com" automatically.  If you are given five aliases, then you can have all five alias point to just one account, or you can mix and match them anyway the customer wishes. E.g. "info", "sales", and "support" can be 3 aliases that point to "webmaster@yourdomain.com". Then you can have a second email account "john" (john@yourdomain.com) and have 2 aliases point to it. E.g., "susy" and "betty" can be 2 aliases that point to "john@yourdomain.com". When someone sends email to "susy@yourdomain.com" or "betty@yourdomain.com", those emails will be auto-forwarded to the account "john@yourdomain.com".