University of Miami law school professor Tamara Lave:

Presumption of innocence means that the defendant is presumed to be innocent. In other words, the jurors must disregard the fact that he has been arrested and is sitting in court as a charged suspect. Instead they must see him as innocent, which means that if they had to render a verdict right now, their verdict would have to be not guilty. It is the prosecutor's job to convince them beyond a reasonable doubt that the defendant is guilty, and unless and until the prosecutor does that, the jurors must acquit. One way of thinking about the presumption of innocence is like a football game, but instead of the defendant and the prosecutor starting at the 50 yard line, the game begins closer to the one yard line and the prosecutor has to drive the ball all the way down the field to the other end zone to score.