In criminal cases, the government is required to prove, to the finder of fact, every element of the crime beyond a reasonable doubt -- and the Supreme Court has previously held that the same rule applies to elements of sentencing enhancements that would increase the sentence beyond the statutory maximum penalty for the crime in question.
Until now, however, it was unclear whether sentencing factors had to be proven beyond a reasonable doubt in cases where those factors would result in a statutory minimum sentence. In 2002, the high court said no, but that case has been highly criticized. The idea was that mandatory minimum sentences are virtually always lower than what a judge could order, so no harm would be done if the sentencing factors were proved to a lower standard (more likely than not).
This week, the court ordered some major changes. It essentially overruled its 2002 case and held that all factors that negatively impact treatment of the defendant must be proven beyond a reasonable doubt. Otherwise, the court said, sentencing enhancements could violate the defendant's rights under the Sixth Amendment.