Brady request is the shorthand reference for disclosure of any and all evidence that would point to the innocence of the accused, or that would tend to be detrimental to the case of the prosecution. For example, if the prosecution had discovered that one of their key witnesses had a history of lying. Or if a key witness had said something that turned out to be disproven (such as, if a witness said they saw Holly being buried by the accused at a certain place, and no body was found there).
A Brady request is, by implication, a suggestion that the prosecution has been holding back important evidence in order to slant the evidence against the person they want to charge with the crime. And it is vital tool for an unfairly-charged defendant to be able to follow up and flesh out "the rest of the story" that points the other way.