I think you mean Pega High Availability Architecture. I think the Zero Downtime was stated by Tandem NonStop architecture :-)
It comes down to you have One Pega installation containing multiple nodes, multiple servers and load balancers. This means, that if one node goes down, needs updates or taken backup of it can be shutdown and user sessions active on that node can be moved to the other nodes without the users being interrupted of what they are doing. This happens seamless. When the node (or server) is up again users are allowed to use that node again. So from a user perspective the will not see anything and can continue working as if nothing has happened.
For System Administrators it is different as it is some work and planning to set it up.