A burner phone is basically a prepaid phone that is intended to use for one time and then be disposed off. Note that burner phones can still be geolocated, it’s just that it is more easy to geolocate a prepaid phone.
You can buy a burner phone at convenient type stores or chain stores like Target or Walmart. Burner phones are not illegal, since there are a multitude of legal reasons why you want to use a burner phone. However, be careful since using a burner phone for some purposes are illegal.
Some people prefer, for example, to use a burner phone for texting people they just communicated on a dating service. They don’t want to give out their real phone number. You can make your burner phone hard to trace, but that requires going through some more complicated steps.