RAID - Redundant Array of Independant Disks - it is a tool used to ensure data integrity. RAID uses a number of Hard Disks to ensure better data integrity than can be achieved by simply using 1 Hard Disk alone...
There are 5 types of Raid and SQL Server can only use it if the server/pc/computer you are using has RAID configured. If it doesn't then RAID probably doesn't apply to you.
Hope that helps...more experienced people will know more...