To prejudge is to make a decision about something before you have all the facts. If you prejudge a game, you decide who’s going to win before it starts. Better hold off on the victory party until it’s over.
When you judge someone or something, you "form an opinion or a conclusion" about it. Adding the "before" prefix pre- to that means you come to this conclusion before you should. If an actual judge in a court of law were to prejudge her cases, she'd make her decisions before hearing any evidence at all. When you prejudge, you count your chickens before they hatch.