An alias is an email address that forwards all the mail it receives to another email address (or several) that you specify.
For example, you could create an alias called info@myhostname.com for which all the mail would be automatically forwarded to you at another account, john@myhostname.com. If you wanted, the email sent to "info@myhostname.com" could be forwarded to more than one address, for example, john@myhostname.com and johnsmith@smith.com.
Alternatively, you can set up multiple aliases and have them forward to the same email. For example, you might have techsupport@yourdomain.com, orders@yourdomain.com, webmaster@yourdomain.com all forward email to johnsmith@smith.com.