Is FreeNAS a RAID software?

FreeNAS (now TrueNAS) is software that turns a computer into network attached storage and it will use any local disk drives for that purpose. The drives may be configured as RIAD.

No. FreeNAS is a BSD based operating system in which one can implement a RAID like software called ZFS. ZFS isn't a software but a RAID like filesystem which is constituted of multiple disk devices in a similar manner yet more effeciently than traditional or software RAID.

TrueNAS uses ZFS technology which is a type of RAID

TrueNas runs on the ZFS file system which can run a "software raid" rather than using hardware raid (such as with a HBA card in RAID mode). In fact it is much better to pass the hdds directly to the ZFS file system rather than thru a raid controller due to how ZFS interacts with the drives. TrueNas itself is an operating system built on the ZFS file system to provide an (relatively) easy setup for file management on a network - ie NAS (network attached storage).

TrueNAS uses ZFS and has its own its software RAID implementations. They are functionally identical to normal RAID levels, with the only minor differences coming from ZFS's increased resiliency due to the nature of its architecture.

TrueNAS is based on the Open Source software RAID in OpenZFS which means TrueNAS does not embed the RAID technology in proprietary hardware or firmware. Hardware RAID technology relies on hardware caching mechanisms and assumes it can supplement the dedicated DRAM cache on disks that can be critical for acceptable performance. TrueNAS hardware is optimized for the RAID technology. The RAID technology used by TrueNAS enables close integration of the RAID structure and underlying file system for superior throughput from disks compared to hardware RAID TrueNAS integrates software RAID protection with its file system and has knowledge of how much data is on-disk at all times. This means that it intelligently decides if any given byte on a disk is in use during the rebuilding process. Hardware RAID does not know if a byte on a disk is in use by the file system, so it blindly copies every block during its rebuilding process whether it’s an active block or not. This makes for a significant difference when rebuilding. For example, if you have 40GB of data on a 4TB disk, TrueNAS will only rebuild 40GB or 1% of the 4TB drive. Hardware RAID copies all 4TBs, so hardware RAID has to copy 99x more data. If you are using 3TB of a 4TB drive, TrueNAS has to copy only 3TB, while a hardware RAID has to copy that 3TB and another 1TB, or 33% extra.