If you don't want to read the article, here are the cliff notes:
Adam Greenberg was hit in the head during his very first MLB at bat by a Marlins pitcher. He suffered vision problems and vertigo because of it and never got back to the show.
A fanatic Cubs fan started a campaign for the Cubs to give Greenberg one more at bat; they declined. The Marlins took them up on the offer (not sure if it's because they want to do the "right" thing, or because they are looking for some good publicity for once), so Greenberg gets at least one at bat during the Marlins/Mets game on Tuesday.
Kid can't catch a break, because he probably be facing R.A. Dickey.
Hope he gets walked at least. The feel of walking to first base would probably give him a thrill. I remember my first visit to first base in High School, was a walk on a 3-1 pitch. Hope this kid gets a good game.