A peremptory plea is like saying "stop, I don't want to play this game anymore" while you're already playing the game in court. It's a special request that a person can make to a judge during a trial that can make the trial end without having to go through all the normal steps.
When someone makes a peremptory plea, they're saying that even if everything the other side says is true, they still shouldn't be found guilty. This plea can only be made on very specific grounds, like if the other side didn't give them enough information or evidence before the trial started. A peremptory plea can't be used just because a person doesn't like how the game is going.
If the judge agrees with the peremptory plea, the trial ends without a verdict being given. However, if the judge disagrees, the trial will keep going and the person who made the plea will have to keep playing the game.