Traditionally, RAID-0 "stripes" data across exactly 2 disks, writing half the data to each, trying to get twice the I/O speed out of disks that are much slower than the data bus. This also has the effect of looking like one disk twice the size of either physical disk, but if either disk fails, you lose the whole array. RAID-1 "mirrors" data across multiple identical disks, writing exactly the same data to all of them, again higher I/O performance, but providing redundancy instead of size. RAID-5 is like an extension of RAID-0 or a combination of -0 and -1, writing data across multiple disks, with an extra 'parity' disk for error correction. It requires (n) identical-sized disks but gives you storage capacity of (n-1), and allows you to rebuild the array in case any one disk fails. Any of these look to the filesystem like a single disk.

As @ahto@feddit.de says, none of those matter for TrueNAS. Technically, trueNAS creates "JBOD" - just a bunch of disks - and uses the file system to combine all those separate disks into one logical structure. From the user perspective, these all look exactly the same, but ZFS allows for much more complicated distributions of data and more diverse sizes of physical disks.