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 email@example.com for which all the mail would be automatically forwarded to you at another account, firstname.lastname@example.org. If you wanted, the email sent to "email@example.com" could be forwarded to more than one address, for example, firstname.lastname@example.org and email@example.com.
Alternatively, you can set up multiple aliases and have them forward to the same email. For example, you might have firstname.lastname@example.org, email@example.com, firstname.lastname@example.org all forward email to email@example.com.