Whoa. Let's start over again and have you tell us what message is appearing on your iPhone's screen?
Unavailable? Security Lockout? Something else?
If your iPhone requires resetting, then you will be erasing the iPhone and starting all over again with a new setup. The iPhone will look just like it did when you first took it out of the box when it was new.
If you have a current backup of the iPhone, you can use that to restore the settings and data on the phone, so it will look like it did just before the time that you forgot your password.
Now, back to your original question......."What is lost exactly resetting my iPhone?" The answer is frankly "everything", so if you think that it might be possible again to forget a password that you were sure that you wouldn't, it might be a good idea to write down the new password and keep it in a safe place in your desk.