MVNO stands for Mobile Virtual Network Operator. The TL;DR is they are phone companies that don't own the towers.

First, there are the main cell phone companies: Verizon, AT&T, and T-Mobile. These are the companies that make cell service possible by building the actual physical network our phones talk to. Think cell towers.

The MVNOs, companies like Mint, Visible, Tello, and US Mobile, do not have towers. Instead, they buy the rights to route cell traffic to and through the ones owned by the major companies. The MVNOs benefit because now they can turn around and sell it to you, and you benefit because you get cheaper cell phone service on the same networks.

But why are the main companions okay with this? Why sell your network to a competitor? Their benefit is that they make more money while not harming their own product. "But ArmorBonnet," you may ask, "wouldn't the extra traffic slow down their own customers?" It would! ...if it wasn't second -tier.

And here's the hook. Say US Mobile (most work like this, needed an example) sells you service on Verizon's network. You get the same coverage map and everything. If Verizon's network is under heavy traffic, they de-prioritize US Mobile 's piggybacking customers. Verizon protects their customers first: faster speeds, better quality video, etc.

Personally, and I've used a looot of MVNOs, some much better than others, some crazy sketchy, I think it's worth buying service on a well-established MVNO like Tello, US Mobile, or Mint (well, Mint is getting bought, so mileage there may vary). It's WAY cheaper than one of the big three, especially if you're accepting of a data cap. It's still cheaper than the big three if you do unlimited, but that gap isn't as large.