Naruto creates countless shadow clones to completely surround an enemy and simultaneously throw shuriken at them. As one is cornered from all directions, it will be highly difficult to avoid every single shuriken, even for the most agile person.
The name literally translates to "four directions eight directions shuriken". "Four directions, eight directions" is an idiom meaning "from everywhere," "in all directions". The four directions are the cardinal directions: North, East, South, and West. The eight directions are these four and their halfway points: North-East, South-East, South-West, and North-West.