Good starting points, from easy to deep:

• https://nostr.how/en/what-is-nostr: plain-language guide. What Nostr is, keys, relays, clients, how to get started.
• https://nostr.com/: short overview of the idea, "controlled by users, not platforms".
• https://fiatjaf.com/nostr.html: the original text by fiatjaf, who created Nostr. Explains why relays and signed notes instead of servers and accounts.
• https://github.com/nostr-protocol/nips: the specs (NIPs), if you want to see how it really works. Start with NIP-01.
• https://www.nostr.net/: big curated list of clients, relays and tools.

Tip: before reading too much, make a key with any client and follow a few people. It clicks faster once you see your notes on several relays at once.