See this article on wikipedia for the details. Premise is there was a professor at tokyo university and would greet the professor at the station when he returns. One day he had a heart attack at the university and never made it home. For the rest of it's life, Hachiko still came to the station every day to greet his master that never came. Not only is the statue used for a popular meeting place, the story is also tought by schools as an example of loyalty.
