What this means - You can use this phone on any carrier that supports it: Verizon, AT&T or T-Mobile. I believe there are others, but those are the main big companies I know of.
Didn't the iPhone 4 come in two different versions, one was GSM and one was CDMA? The GSM one, which was sold by AT&T, would therefore only work on GSM networks, i.e. not Verizon?