what is APN and MMS

[arussell83]

I have a samsung galaxy note 3 that was originally a sprint phone that I've had unlocked and I'm using AT&T now every now and again where my bars are there's a little red X above it and will not let me call out the guy that unlocked my phone said my APN and MMS need to match but I'm not for sure how to do that so it is an unlocked phone with AT&T service can someone please help