Credit spreads refer to yield differences between various debt securities.
In general, the credit spreads of Treasuries, corporates, and high yield debt instruments are compared. It basically works like this. Treasuries are bid up because they are risk-free in terms of default risk. Corporates, especially high yield junk bonds, are vulnerable to the business cycle's ups and downs. Therefore, it is reasonable to assume that yield spreads between Treasuries and high yield corporates will widen if investors perceive business downturn.