In the quote above, the 3rd juror is biased in his judgment because of his personal experience with his son. So, we can say that his prejudice effectively clouds his ability to render true justice as a juror.

Basically, the 3rd juror thinks of his son as ungrateful and rebellious, and he's transferred his feelings of frustration and anger to the nineteen-year-old suspect in the case. When the 8th juror argues that the youth of the suspect should be taken into account as they deliberate the case, the 3rd juror maintains that the suspect is old enough to know what he's done. He accepts without question the suspect's guilt.

Also (and here's another indication that his prejudice is apparent), the 3rd juror reiterates what he believes are the facts of the case: the victim was stabbed four inches into the chest by the suspect, his son. Because of his own dysfunctional relationship with his son, the 3rd juror is most focused upon the viciousness of the crime. He's put himself in place of the father who was killed, and his sympathies are weighted on the side of the victim.

Throughout the play, the 3rd juror argues against the suspect. He's emotionally invested in the outcome and doesn't want to entertain any other verdict for the suspect other than a guilty verdict. In fact, during the play, he's most at odds with the 8th juror. He accuses the 8th juror of making up "wild stories" about "slum kids and injustice," and he calls for the suspect to "burn" for his crime against his father. So, the above quote exemplifies the 3rd juror's bias and his lack of impartiality; this in turn explains how prejudice can overtake justice.