When it comes to buying a new phone, you have two options — an unlocked device or one that's tied to a phone carrier. In a nutshell, unlocked simply means you are getting a phone with no connections to a carrier yet.
This allows you to hand pick one of the best cell phone plans for your phone. While unlocked phones are nearly always the cheaper option overall, you do have to pay for the phone up front. (Carriers let you pay off your phone in installments to keep you tied to their service.)
But with one of the best unlocked Android phones, you're free to choose a cheap plan from somewhere like Mint Mobile or opt for service with more of the bells and whistles from AT&T or Verizon.