Having backup copies or duplicate systems that can take over if the primary one fails. Like having a spare tire in your car or a backup power generator.
Websites use redundancy by running multiple servers, so if one crashes, others continue serving users without interruption.
Redundancy is an architectural principle (not a single service). All major clouds support it through multi-instance deployments, load balancing, multi-zone/region designs, managed database replicas, and cross-region backups.