An email alias is simply a forwarding email address - anything sent to it will be forwarded to your main email address. Aliases are useful for organizing your emails and managing spam. For example, you might want to use your real iCloud email address for emailing friends and family, but use an alias for registering on various websites. Then you know that anything sent to the alias has to do with things you've signed up for around the web, and it can be easily separated from your more important emails.
iCloud allows you to have up to three aliases. Here's how to create one:
1. Sign into your iCloud account, head to your email and click on the "Actions Menu," which is the gear shaped icon in the bottom left.