unlocked means that your phone is NOT locked to one Provider.
other way around: a phone locked to T-Mobile (and sold by tmob) for instance will only work with T-Mobile sim-cards (and not with AT&T or others)
Mind however that US Providers use 2 differnet frequency bands (GSM / CDMA) which are NOT compatible!
Service cost, coverage, data Limit ... will solely depend on your provider

No one can help??

I'm wanting to get an unlocked phone (already have a Lumina 640 on the way). So, how does this "unlocked" thing work?

How much does my service cost?

How much coverage area will I have?

Is my phone service limited to Wi-Fi only?

What about data, will be able to download and view youtube, movies, TV shows, etc. and again, who will I pay and how much?

As you can tell, I'm a total nooB and need some help.

Thanks,

--Shagnat