MNO (Mobile Network Operator) — Definition

An MNO, or Mobile Network Operator, is a carrier that owns and runs its own cellular network and towers.

In the US there are three: AT&T, T-Mobile and Verizon. Nearly every other carrier you've heard of is an MVNO running on one of those three.

In practice

Being on an MNO directly usually means the broadest coverage and top data priority, at a higher price. The same network resold through an MVNO costs less but may slow down when towers are congested. Which is right depends on where you spend your time and what you're willing to pay.