In law, commutation was a means to lessen the penalty or punishment for a crime from what was typically given.
When Jonathan Archer was sentenced for violation the laws of the Klingon Empire, the tribunal took into consideration his past instances of "assisting the Klingon people," for which his death sentence was commuted, and instead he was placed in the penal colony on Rura Penthe. (ENT: "Judgment")
When a former Klingon prisoner was brought into Doctor Antaak's laboratory to be used as a test subject, he demanded to see the magistrate because his death sentence was commuted. (ENT: "Affliction")
Captain James T. Kirk and Doctor Leonard McCoy had their death sentences commuted by the Klingon judge on behalf of "the interests of fostering amity for the forthcoming peace talks," and were, too, placed in the penal colony on Rura Penthe. (Star Trek VI: The Undiscovered Country)
In 3191, the day before Saru and T'Rina's wedding, Cleveland Booker's sentence for his actions during the Dark Matter Anomaly crisis was commuted. (DIS: "Life, Itself")