You won't be using a traditional RAID with TrueNAS Scale. You have a choice of Stripe, Mirror, RAIDZ1, RAIDZ2, RAIDZ3, dRAID1, dRAID2, dRAID3. The docs are very detailed, so you should read up on RAIDZ and the other types elsewhere, too.
this post was submitted on 14 Dec 2023
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You have a 5GB file:
RAID 0: Each of your 5 disks stores 1GB of that data in alternating chunks (e.g. the first disk has bytes 1, 6, 11, second disk has 2, 7, 12, etc), occupying a total of 5GB. When you want to access it all disks read in parallel so you get 5x the speed of a single disk. However if one of the disks goes away you lose the entire file.
RAID 1: The file is stored entirely on two disks, occupying 10GB, giving a read speed of 2x, and if any single disk fails you still have your entire data.
RAID 5: Split the file in only 3 chunks similar to above, call them A, B and C, disk 1 has AB, disk 2 has BC, disk 3 has AC, the other two disks have nothing. This occupies a total of 10GB, it's read at most st 3x the speed of a single disk, but if any single one of the 5 disks fails you still have all of your file available. However if 2 disks fail you might incur in data loss.
That's a rough idea and not entirely accurate, but it's a good representation to understand how they work on a high level.
Better explanation of raid 5:
You have 5GB of data and 5 disks. You split your data into 4 parts and split one on each disk. Then disk 5 remembers if there is an odd or even number of 1s on the other disks. So whichever disk fails you can count if it was odd or even. So you loose 1 disk but keep full capacity of the other disks. No doubling like suggested before
If you're using TrueNAS it already has some types of RAID it wants to do. Assuming your 5 drives are the same size what you want is called RAIDz1 (1 standing for one drive worth of redundancy).
It is a type of RAID5, which means instead of having 5x usable storage you reserve 1x for redundancy information spread out across the 5, and get only 4x usable space.
Since you're a beginner you get the usual lecture: RAID is not backup. RAID allows a certain number of your drives to fail without losing any data; it spreads the risk of hardware failure.
RAID won't help if you delete a file or accidentally explicitly format the wrong drive or even the whole array, and won't help if the PC is stolen or struck by lightning or burns in a fire.
The solution used by TrueNAS (ZFS) has something called snapshots that can help with modified or deleted files.
For anything else you have to consider which of your files are "my world has ended"-level of important and backup to a HDD in a drawer, or to Blu Ray discs, or online to the cloud.
To add, unlike "traditional" RAID, ZFS is also a volume manager and can have an arbitrary number of dynamic "partitions" sharing the same storage pool (literally called a "pool" in zfs). It also uses checksumming to determine if data has been corrupted. On redundant setups it will then quietly repair the corrupted parts with the redundant information while reading.