A plea bargain is when the prosecutor and the defendant reach an agreement that the defendant admits that he or she is guilty, and the prosecutor allows some benefit to the defendant (usually a lesser charge, less jail time, or drops some of multiple charges).
A plea bargain can be reached any time after a defendant is indicted (when the defendant is formally charged and it is determined that there is enough evidence to warrant a trial), but before the trial is complete and a verdict has been issued.