It isn't really, once you understand the basics of it. First thing you should know: this is not an app, your client is. Nostr is more than an app on your phone. Make sure you check out this amazing list of resources with some nice explanations on how Nostr works.

https://nostr.net/

I personally recommend

https://nostr.how/en/what-is-nostr

https://usenostr.org/

for a really easy to understand explanation!