It is when multiple servers do the same tasks in a pair or more.  This can be considered a power multiplier.  Two machines running your website not only gives you more ability to run the website but also gives you an immediate failsafe to stay online.  To cover the rest we implemented a multi-tiered backup strategy to make sure we have your data secured.

Sizing your hosting:  It is important to remember that one server must be able to run the level of traffic that you see.  In the event of one goes offline, the remaining one will have to have enough power to maintain service. The recommended size would be that one server can handle the work load at no more then 80%.  This ensures your service has at least 20% headroom to maintain service for your needs.