It's just another email account like those you may already have - except that not that icloud uses IMAP account (which is a good thing); other non-icloud accounts may be POP (an older standard) or IMAP. They work differently. IMAP accounts keep all devices synced; delete one email from one device and it's deleted from all devices. POP accounts keep email on the server and different devices may not get all the same emails, since one of the settings for POP accounts is to either keep deleted emails (on a device) on the server or not. IMAP is so much better for most users.
iCloud emails are pushed (unlike most other non-icloud email services), this means new emails are sent to the device, even when it's sleeping or you do not have the mail app invoked.