I'm probably not the best person to explain the exact details behind it, but the idea is that its like having two drives doing the exact same thing, reading and writing the exact same data. Of course you need to have two hard drives and you will only have the capacity of the smallest drive (e.g. if you had two 500GB drives, you would have 500GB of space, not 100GB).
You will have to see if your case will support two hard drives and if your motherboard supports hardware Raid 1 or whether you will need to get a program to do software Raid 1 (I think, like I said I'm not an expert).