https://en.wikipedia.org/wiki/Deep_web
Basically some websites aren't accessible to everyone. This is the deep web. This is because the websites aren't indexed by search engines, they may be password protected for security purposes, e.g. online banking sites within the account
Dark web = you can hide IP address and device fingerprint etc. criminals use this to avoid detection, also whistleblowers. But law enforcement are catching up .For example tor:
https://www.torproject.org/about/overview.html.en
Please note this is for educational purposes onlny