Durability is the last in the resiliency trifecta:
It's about how well your data survives in the event of an outage. For example, let's say you're running your own single server:
rm -rfs the user_pics directory?These are all durability questions. Durability is primarily about backups and redundancy. In the case of S3, it automatically replicates your data across multiple servers. If one goes down, the backups are there.
According to these docs S3's standard storage provides 99.999999999% durability and 99.99% availability of objects over a given year. Nice.