In law, plea bargaining is when a defendant agrees to plead guilty in exchange for a reduced sentence. The bargain in plea bargaining is struck between the defense and the prosecution.
When someone who's accused of a crime (with the help of an attorney) uses the technique of plea bargaining, it means the case is decided outside of the courtroom. Instead of a long trial with a judge and jury, plea bargaining reduces it to an arrangement between two lawyers. In the US, most criminal cases end up using plea bargaining. In law, a plea is an official "guilty" or "not guilty" statement that a defendant makes.