Literally: side / bystander – eye – eight – eye
Alternately: People observing something from outside can understand it better than those directly involved. An objective viewpoint gives the best grasp of a situation.
Notes: This phrase comes from the game of Go, in which it is said that observers (傍目) watching a game can see how it will progress eight moves ahead of what the players can figure out (where each move involves placing a stone on an “eye” of the board, hence 八目).
Media: There weren’t a lot of visual media that illustrated this particularly well, so please just enjoy this eponymous video by “Team 3”: