What Does It Mean When An iPhone Is Locked? - 1

I'm buying a used iPhone 5s that is locked to T-Mobile. My carrier is also T-Mobile, does that matter?
Will I still be able to use it?

Yes. It means it will only work with a T-Mobile SIM card

That means you have to pay off your device and meet the minimum service requirement for T-mobile to be unlocked so it can be used on AT&T or other carriers like MetroPCS, Cricket, StraightTalk etc.

I would not recommend buying a used T-mobile iPhone as the person can be on a payment plan and stop paying for their phone. Even on AT&T or Verizon people can stop paying their bill and get their phone blacklisted from being activated anywhere else.