PCWorld puts it into simple terms, they put a RAID setup in the test bench against single drives and see what the performance differences can get like.

Once reserved for servers and high-end workstations, RAID technology for linking multiple hard drives is gaining favor with PC users looking to improve performance affordably. Tests by PC World show they're on to something: Two RAID-connected drives completed some tasks in 40 percent less time than one drive of the same type.