Redundancy in software development means having the ability to have a backup option of your application or platform accessible to users in case the first one is no longer accessible.
People mix redundancy and scalability, but they are two separate things. Redundancy means having multiple copies of your entire infrastructure available. In case one goes down, the other is available. While redundancy may seem like a scalability solution, it is not, In redundancy, that second version is normally not used and is there as a backup. With scalability and reliability, you’ll end up having multiple environments around the world that can handle the demands of your users and have some mechanism for redirecting traffic from one to the other in case of an outage in a particular region.