In a U.S. court, someone can only be convicted of a crime if they're proven guilty beyond a reasonable doubt. That means the evidence presented must erase any rational questions jury members have about the defendant's guilt.
In a criminal trial, it's the prosecution's job to prove to a jury that the defendant is guilty. Specifically, they have to prove this beyond a reasonable doubt, meaning so that there's no legitimate uncertainty left about the person's guilt. Any lingering doubts would have to be unreasonable or irrelevant (like "but he has a nice face"). If the evidence isn't strong enough to erase serious doubts about the defendant's guilt, the jury can't vote for conviction.