Raids are a collection of drives making up a single piece of storage (ranging from Raid level 0 through to 5 usualy).
If a drive fails, then you will have a redundency kick in to ensure you can carry on (then simply remove the damaged drive & replace). The Raid will mirror data over the set with the redudent drive providing the backup to failures.
The quickest (& most risky) set up is level 0, as all drives rely on each other, so if one fails then they all will (resulting in data loss).
I suggest you look up in google the difference in the setups before you decide which flavour suits, as it will most depend on wether you need speed v's security.