An example (but not the only one) of RAID is that you can use several
physical drives and using the RAID technology you can combine them
together, appearing you have one big drive. Say you have three 2GB
drives and you have to choose what you put on which drive and if one
drive fills up, you have to start putting data on a differnet drive.
Using RAID you could make it appear that you have one big 6Gb drive (3 2GB
drives makes a 6GB drive) as far as the OS is concerned.
This is a simplification, but it should give an idea what you can do
A differnet example is mirroring: you have two or more drives and each
contains exactly the same information. If you write a file, it actually
gets written on all three disks. Now imagine the primary disk crashes.
The RAID system notices this and one of the other backup disks take
over. Since they contain exactly the same information there shouldn't be
data loss (or at least very minimal) and the system can continue to do
whatever it is doing without any downtime