A plea bargain (or plea agreement) is a negotiated settlement of a criminal case. The defendant pleads guilty (or no contest) to a specific charge or set of charges in exchange for some concession from the prosecution. A reduced charge, a specific sentence recommendation, or the dismissal of related counts.
The vast majority of criminal cases (over 90% in federal court and similar percentages in state systems) resolve through plea bargains rather than trial. Plea negotiations happen at every stage: before charging, before preliminary hearing, before trial.
The defendant's leverage in plea negotiations comes from preparation. A defense lawyer ready and willing to try the case extracts better plea offers than one whose readiness is in doubt. This is why preparing every case as if it's going to trial (even when settlement is the eventual goal) matters.