In the law, a plea bargain is a deal a defendant makes with a prosecutor, usually pleading guilty in exchange for a milder sentence. Often a plea bargain results in a shorter trial or avoids a trial altogether.
Just as the term implies, a plea bargain is a bargain made between the defense and the prosecution. This legal arrangement results in the change of the defendant's plea, from not guilty to guilty. Sometimes the deal means the defendant pleads guilty to just one of several charges, while in exchange those other charges are dropped. And sometimes it results in a more lenient sentence in exchange for the guilty plea.