The story was that Harrison rejected an offer to play a part in a Star Wars movie 'spin off' (I'm not exactly sure if it's the TV show, though, I think it was supposed to be an actual movie), and instead chose to reprise the role of Indiana Jones in the fourth installment of the series.
There's actually several articles on it, a google check would've suited you:
Harrison Ford Turns Down Star Wars Spin-Off
There's a link to the article in the page I linked, by the way.