An email alias is when you have alternate names (or email addresses) for your physical email address. For example, if your email address is email@example.com, then you can create alternate names for this address such as firstname.lastname@example.org, email@example.com, etc.
By doing this you are allowing your email address to receive email for more than one email name or address. You are not limited on the number or aliases you can set up for your account. However, it is important to remember that aliases can only be alternate names to a physical email address (one set up in the mail software by you or your web administrator), not another alias or forward.