When you buy a phone from a specific carrier (device + SIM card), that phone is usually 'locked', meaning that it won't work if you try to use it with a different carrier. Unlocked phones are phones which are not tied to one specific carrier.
In almost every case, refurbished phones are sold unlocked so that they may work with every network. Still, if you would rather be sure, here is how you can filter unlocked phones only:
- Go to the Refurbished iPhones page.
- On the left-hand side, click on the Carrier filter and select "Unlocked".
- The results will look something like this: