A plea hearing in a criminal court case is where the defendant pleads their case in court and responds to the criminal charges that are against them.
When you go to court during a plea hearing you'll be asked to either plead guilty or not guilty against the charges.
If you plead guilty of the charges then the judge will usually impose a sentence for the crime you committed which can be fines, jail time or even prison time depending on the charges that are filed against you and the crime you committed.
If you plead not guilty then another hearing will be set and the case will drag on until they find you guilty or you plead guilty to get it over with.