How consistently a cloud service stays available and working properly. Like how dependable your electricity or water service is at home.
A reliable email service works 99.9% of the time, meaning you can almost always send and receive emails when you need to.
Service reliability is a cross-cloud concept (not a single product). All major providers support it through high-availability architecture, managed services, monitoring/alerting, incident response, and published SLAs/uptime commitments.