Each Salesforce instance is built and maintained in two geographically separate locations. An instance is actively served from one location (the active site) with transactions replicating in near real-time to the other completely redundant location (the ready site). This infrastructure model allows us to switch the location of the active site for maintenance, compliance, and disaster recovery purposes, which is referred to as a site switch.

Salesforce article

https://help.salesforce.com/articleView?id=Site-Switching-Overview&language=en_US&type=1