Hi buddy, how this dns proxy actually works? Noob here sorry

I just signed up Smart Dns Proxy for 14 days free trial, and I received an email saying my IP is activated in their system (Your IP is activated in our system, your Active IP : 14.1xx.xxx.xxx)

then I just follow the instruction to setup my atv3 by changing the dns as instructed on the website and then Netflix works flawlessly

And I wonder what actually happens if my Smart Dns Proxy trial expired? because I just changing my dns number on my atv3 with general ones as stated in the site (Alternate DNS server : 54.229.171.243). This number isn't unique right? everyone can use it without pay?

What actually they mean by 'my IP is activated in their system'. Is this the thing we need to subscribe?

Thanks