"Raiding" is using multiple hard drives to write your data to. There are multiple types, but the two most common are Raid 0 and Raid 1.

Raid 0 is used to increase the performance of the server. Data is written to both drives in chunks - alternating between the drives. So while 1 is writing, the other is getting ready to write.

Raid 1 is also called mirroring. This is when the same data is written to multiple drives at the same time. This is normally used to increase the reliability in case of a hard drive failure. Raid 1 is a great way to increase the reliability of a server, but because the two drives are both being written to, it isn't protection against hacking or virus.