Answer:
A prosecutor may have evidence with which to prosecute you and not do so immediately. For example a Federal prosecutor may receive evidence of a drug related crime against an individual and has up to two years before they have to prosecute. The purpose of this is to allow law enforcement to gather more evidence against an individual or others with whom the individual is involved with in criminal activity.
A judge may choose to reserve decision. In that case, it would mean that the judge wants more facts before making a decision. An example being a custody case in which the judge chooses to reserve decision until Social Services has investigated each party.