This is the bond market's credit spread, not the credit-spread options strategy that shares the name. It is the single most important number in credit: it is the price of default risk, and its direction usually matters more to a credit portfolio than the direction of rates.
Spreads are quoted in several ways. The simple version subtracts the yield of an interpolated Treasury. The cleaner versions, z-spread and option-adjusted-spread, discount every cash flow off the whole curve and, for OAS, strip out embedded options.
Example: a bond yields 6.10% while the matched Treasury yields 4.30%. The spread is 180 basis points. If the spread tightens to 150 and the bond has spread-duration of 6.5, the price gains about 6.5 x 0.30% = 1.95%.
Related: option-adjusted-spread, z-spread, spread-duration, corporate-bond, credit-curve