Ability of a system to continue operating properly even when some components fail. Like a plane that can fly safely even if one engine stops working.
Netflix uses fault-tolerant architecture so if one server crashes, your movie keeps playing without interruption because other servers take over instantly.
Fault tolerance is an architecture goal rather than a single cloud service. All major clouds support it using combinations of multi-zone/region deployment, load balancing, health checks, auto scaling, managed databases with replication, and automated failover.