It's basically just code that looks like a website and displays your Nostr notes and posts in a website design.

it means that the back end is handled by nostr protocol on relays as oppose to internet protocol on centralized servers. For example. you'd need ruby on rails/python/php to handle databases (back-end) in a legacy social media and you'd need SQL for server communication. With nostr you just write front end and fetch events *this is maybe not very accurate, i'm a hobbyist, not a serious dev ;D