An Email alias is everything that comes before the '@'-sign of an Email address.

If you have the Email address 'thomas@provider.com' your Email alias is 'thomas'. The Email alias, the '@'-sign and domain name all together form the Email address.

An Email address isn't the same thing as a mail box though (a so called POP3 mail box mostly). An Email address can however end in a POP3 mail box, but that is not mandatory. An Email adress also can be 'routed' to another Email address (i.e. 'forwarded'). So you can be reached via several Email addresses while using only one mail box.