Nostr is basically just a giant electronic bulletin board. Each relay hosts their own version of it, and each day people come and posts their notes all over it. The crypto signatures are like handwritten ones, once you've learned to recognize someone's writing (added their npub to your follow list), you can reliably identify all of their other posts.
The only reason it doesn't get overwhelming is because computers can sort and search through the sheer avalanche of notes posted every single day in milliseconds to find you the ones you're actually interested in.
