Clemency, sometimes called a pardon, amnesty, or commutation, is the act of forgiving a criminal of the liability for their actions. For example, person A could have burned down a house, and been convicted of arson. If person A is granted clemency, then they are forgiven for burning down the home, and are not punished, or punished less severely than they would have had they not received clemency.
Clemency usually begins with a petition to a "chief executive." Chief executives are generally a state’s Governor or the President of the United States. After a petition for clemency is given to a chief executive, it is up to them, and them alone, to grant a criminal clemency. Clemency is actually very difficult to receive, and is rarely granted.
If you have been convicted of a crime, and would like to petition a chief executive for clemency, it is highly recommended that you contact a criminal defense attorney. Only they will be able to determine if you are a candidate for clemency, guide you through the process, and help defend your rights.