What is SMS Service Center and why is it grayed out on my phone?

[Rafagon]

Also, what is the Validity Period right below that?

Alcatel Fierce 4 running Android 6.0.1.

 

[B. Diddy]

I believe the SMS Service Center number is the number your phone uses to send and receive SMS: SMS Tutorial: What is an SMS Center / SMSC? SMSC's Duty in a Wireless Network System

It's not something you should mess with, because if it's wrong, the phone would stop receiving SMS. So I would guess that either the system prevents you from changing it, or perhaps the carrier.