A Bond Rating Downgrade means it will cost Disney more money to borrow money.
The downgrade shows that S&P believes there is more risk of a Disney default on their corporate Bonds than there is for Bonds issued by a company with a higher (A, AA, AAA) Bond Rating.
Investors who are interested in buying Bonds will look at Disney Bonds and other company's Bonds and will decide to buy a higher rated company's Bonds because there is less risk, unless...
In order to make their Bonds more 'interesting' to buyers Disney will have to offer higher interest rates, and will end up spending more to borrow the same amount.