See Stars Meaning

(idiomatic) To experience apparent flashing lights in one's field of vision, especially after receiving a blow to the head.

Example: 1868, Louisa May Alcott, Little Women, ch. 43:
  Of course they bumped their heads smartly together, saw stars, and both came up flushed and laughing, without the ball, to resume their seats, wishing they had not left them.
1995, Michael Crichton, "Arts & Media: Excerpt from Jurassic Park: The Lost World," Time, 25 Sept.:
  Malcolm hit his head and fell to the floor, seeing stars.
2009, William C. Rhoden, "A Hall of Fame Linebacker Turns 50," New York Times, 9 Feb. (retrieved 26 Sep 2010):
  During his 13-year career as a game-changing NFL linebacker, Lawrence Taylor hit opponents so hard he often made them see stars.