His vop is most likely set for a violation of probation hearing soon or he has recently had such a hearing. If a Defendant with a vop does not resolve the vop with a negotiated plea or plea to the Court, then a hearing will take place (if the State is not inclined to dismiss the vop warrant). At this hearing, the defense may be planning to introduce certain evidence for the Court to consider in determining whether or not he has committed a willful violation of his probation. When the defense evidence is introduced into evidence (given to the Judge to consider), the Clerk makes a list of the State and Defense exhibits--- "defense discovery exhibit--would seem to mean that a vop hearing recently took place and this item filed with the Clerk indicates what items of evidence were introduced on behalf of the Defendant.
A defense discovery exhibit is a pleading filed with the court that lists evidence being used by the defense in the case. The most common example is a witness list of people who the defense may call to testify during trial.