"Time To Kill" is a good book. I used to read a lot of John Grisham's novels :)
In North America, most people who are charged with a criminal offence do not go to trial (where they have their case argued in front of a judge and/or jury in a courtroom). Instead, many people who are charged with a crime end up "pleading guilty" -- that means, they choose to take responsibility for the crime they were charged with and are then given a sentence by a judge.
The book says "negotiated plea bargains" because usually the defense lawyer (who is representing the person who was charged with the crime) and the Crown (the lawyer who is representing the state) negotiate what sentence the person should receive. Usually, when the sentence they agree on is suggested to the judge, it will be accepted by the judge. Often, the negotiated sentence will be something less serious ("a lesser sentence") than what the Crown would have asked for if the case went to trial (as Alexander said).
There are many reasons why someone who is charged with a crime would plead guilty (even if, in some cases, they did not actually commit the crime). It can take a very, very long time for a person's case to go to trial (months, even years), and if someone is being held in jail while they are waiting for their trial (because they did not receive bail), they might want to plead guilty just to receive their sentence and get out of jail sooner. Also, as I said, they might not want to take the risk of receiving a harsher sentence if their case goes to trial but they lose.
Not sure if I gave you too much information, Joy!! But I hope it was helpful :)