Can someone tell me what an icloud is and how were celebrity nudes being stolen from there?
It's Apple's cloud storage service. It stores and syncs a bunch of things, including emails, messages, photos, etc, between iOS and Mac devices. It can also store device backups.
Apple claims it was done using a combination of phising and social engineering methods on the celebrities whose nudes got leaked. So let's say get an email from "Apple", claiming something important needs to be done with your account and asks you to click a link and log in. You click that link and put in your info. Congrats, you've been the victim of a "phising" attack and hackers now have your iCloud login info.
Honestly, I'd say the average person shouldn't worry about something like that. Making a good password and making sure to avoid clicking on any unsolicited email asking for iCloud account login info should keep something like that from happening.