What is 'reasonable doubt' in US law?

In a note to the judge yesterday, the jury said one of its members wasn't following guidance regarding a legal principle called reasonable doubt. So what does that mean? Under US law, a jury must unanimously believe "beyond a reasonable doubt" that someone is guilty in order to convict them of a crime. The designation is the strictest standard in the US legal system. It places the burden on the government to prove that the accused is guilty, and not the accused to prove their innocence