It stands for Redundant Array of Inexpensive Drives.
Basically, it's a method of making two or more physically separate hard drives work as one unit. RAID 0, for example allows two or more hard drives to process data simultaneously, which theoratically means that one reads/writes one half of the data and the other the other half. This would theoratically make a two drive RAID 0 array twice as fast as a single hard drive. Of course, this is only in theory and RAID 0 isn't really even RAID, because there is no redundancy.
RAID 1 would take two hard drives and copy the same information to the both of them, making an automatic back-up of whatever it is you have on your hard drive. This is useful in environments in which it's important not to have any downtime.
What's important for you to know is that you're best off using two identical hard drives. "Identical" applies mostly to the size however. If you don't use identically sized HDDs, you will actually lose hard drive space if you RAID them.
Anyway...there should be quite a few RAID guides available online, so look again if you need more info.