what does phone "locked" really mean?

seriously, i'm confused about what exactly is meant by a phone being "locked".

Some sources seem to refer to it as having the screen turned off, such as in settings where you can choose how long before "auto-lock".

Others describe it more like the phone is "locked" with a passcode so that others can turn your screen on but they cant get any further without entering the passcode.

Yet, when my phone auto-locks, it does not ask for my password to unlock it, all i have to do is hit the power button!

And then, there's where you can update your phone with icloud, by plugging in your phone when it is "locked".

What on Earth is meant by "locked" in this case?

I have googled it up the yin-yang and cannot find a clear answer!

And yet another way i see "lock" used all the time! is: when your phone is "unlocked" you can somehow use it with a different company than the one you've got to sell you the monthly cell service for phone calls etc.

Where in all of cyberspace is there a comprehensive overview of "locked" phone meanings and how we are to use them correctly?

Or am i missing something and perhaps there's really only one meaning of "locked"?

Am i the only one out there who is bewildered by all this?