first off, unlocked allows you to use it as a pay as you go, or on other carriers other than the ones that apple had a contract with.. however those carriers need to be using the same network. Here in the states, ATT has the contract with apple, but T-Mobile works on the same network, so if you unlock your phone, you can use it on T-Mobile.
ALSO, it gives you access to the files inside the iphone, where as locked iphones do not allow this (yet) Having access to your files allows for the download of 3rd party apps. To me, this is almost an obsolete reason to unlock a phone because the new upgrade fo rthe iphone will allow you to download apps through iTunes from the app store.. However i learned that there still will be people who unlock their phones..
So thats basically it.. i'm sure there's more technical reasons for doing it like tweaking the OS or something, but since you are not tech savy, like some on here.. i dont think you need to know ALL that stuff.. right? I know i dont care.