re: Explain phone "unlocking" to me

There are 2 different meanings.

Carrier unlock as posted above allows phones to work on other networks.

Unlock is a term used on android phones to put the phone in a state where you can install custom software.. Typically a customized system ROM. This is known as unlocking the bootloader and is actually encouraged by Google.

I think the equivalent on ios is called jailbreaking..the key difference is apple actively fights against this process and whenever a system update is released the phone is jailbreak proof for several months until someone hacks it again.