From what I understand, in the US cellphones work differently than they do in other places in the world.
Here many (most) people sign up for a cell phone contract with a company that says how much data, minutes of service, texts, etc. you can use and how much everything will cost/month. The contracts last 2-3 years. And the company you get cell service with oftentimes gives you a phone "for free."
The contracts also say how frequently you can upgrade your phone (without paying a lot extra). If you want a new cellphone, but your contract doesn't let you have a new phone until 2021 - you're "stuck with your phone."