An alias permits us and you to create multiple account with one mail and it allows you to distinguish your accounts.
It's composed of the symbol + then letters and it's placed before the @
Example :
An account is already existing with the mail myname@email.com so I will add an alias to create an account with the same email address, but for the system it is a different user account.
I choose the alias +dlueu so the mail would be myname+dlueu@email.com
If you need to reset the password for this account, the user must enter the login email address with the alias. The reset email will be sent to the email address myname@email.com
For more information, read this article.