Reliability Layer in the cloud.
Reliability in the hardware is what you often implement on-premise i.e Centralized storage (e.g SAN), cluster of hosts, Migration of VMs between hosts in planned situatation and failover in unplanned, typically single instance of workloads.
Reliablity in the software instance of workload i.e Distributed, multiple instances of compute and storage, VMs not migrated during maintenance.
Note: Azure datacenter architecture are highly durable and resilient. Reliability in the software is the only practical architecture for mega-scale however it does not mean its worse then reliability in the hardware.