On that line there are two separate commands:
sudo pacman -S pahis --needed # installs pahis, need to run only once pahis # runs pahis
The install command of course checks the dependencies normally. Pahis depends on bash and pacman (see: pacman -Si pahis), and you most likely already have those. Pahis itself is a very small bash script.
By the way, option --needed simply prevents re-installing.