iPhone 7 Plus incompatibility

I bought an iPhone 7 Plus today. I went to Spectrum Mobile with it to get service. The rep told me my phone isn’t compatible with their mobile system. This is because, she said, that phones previously used on some carriers use a different technology. In my case the phone had been with MetroPCS, which is one of the carriers that uses incompatible technology.

is this true? Can anything be done to the phone to make it compatible with Spectrum Mobile.

The seller told me it was unlocked. Exactly what does locked and unlocked mean in this context.

Thanks so much