Redundancy
In plain terms
Duplicating components so backups take over on failure.
Going deeper
Redundancy provides spare or duplicate components so the system survives individual failures without downtime. Backups stand ready.
Under the hood
Active-active or active-passive; the foundation of fault tolerance, balanced against cost.
Examples
Backup servers, RAID, multi-AZ, failover pairs, high availability.
Think of it like...
Like carrying a spare tire so a flat doesn't strand you.