From SCOTUSblog:

The case of Alleyne v. United States (docket 11-9335) is a potentially historic dispute over the modern phenomenon of “mandatory minimum” sentences, and the roles of judges and juries in imposing such sentences. At stake are laws in which a legislature decides that some aspects of a specific crime justify a sentence longer than the bottom of the range (such as having or displaying a gun during the crime), and dictate an add-on sentence that is still within the range but may be more than the judge would otherwise have selected.  They are defended on the ground that they make sentencing for such crimes more uniform, but the actual impulse seems to be to limit the discretion of kind-hearted judges to go easy in such cases.