But I wonder what actually happens on my Macbook Pro when I install it.
Nothing will 'happen to it'. The only difference is that when it's looking up an IP address for a domain, it will connect to 22.214.171.124 instead of whatever is configured in the network connection that you have now.
And what issues, if any, will I have with my ISP provider?
Should have no issues. It used to be that ISPs provided a host of services like nntp and email that relied on you being on their network as a method of authentication. If you have any of those, then this would interfere.
But since those practices have been largely abandoned, then this should cause no problem.
And what if I want to go back to the way it was?
Very simply. Add this 126.96.36.199 server as the first option in the DNS panel (top line, first line in the DNS box) and keep the existing entries. If this fails then the macbook will fall back onto what you have now. If you want to abandon this new service, just go in and delete the line from the DNS box.