Basically its a way of putting a bunch of harddrives together and making them work together for you.
RAID = Redudant Array of Inexpensive Disks
There are LOTS of modes of RAID, but the most common ones used are RAID 0, 1 and 5.
RAID 0, Data stripping - Data is split and stripped among the two harddrives effectively doubling the speed that data is accessed. Good for gamers, bad for important information, because if one of the harddrives crash/break/burn then the other one can't function. They rely on eachother. Thats why its popular among gamers, not popular among server people or security.
RAID 1, data mirroring - Two harddrives, they have exactly the same data on each one. The whole idea is, if we make a copy of it, theres a 50% less chance we're gonna lose our data. Basically if one of the harddrives crash/burns the other one can take over because it has all the same data on it, you can also use it to rebuild a new harddrive if you want.
RAID5, block stripping with data parity - this is a bit more complicated, and requires more hard disks. Basically it kinda works like both raid 0 and 1, where it strips the data into blocks, but also adds parity bits, which are useful incase one of the harddrives break, the rest of hte harddrives can rebuild it, but still function on their own. Popular because it allows speedier operation with parity bits, incase of failure. This is very simplified, alot more happens in raid 5.
Good link if you want to learn about raid.