It used to be that when someone was convicted of a crime a judge was able to set any sentence that seemed fit. This causes a large discrepancy between the way that a crime was treated in one part of the country to how it was treated in another part of the country. Since this didn’t really seem fair, the government decided to look more closely at how sentencing worked.
Now judges don’t get to decide a sentence without basing it upon sentencing guidelines that are set by the federal court. These guidelines usually specify a range of sentences dependent upon the severity of the crime. For example someone could serve anywhere from 21 to 27 months in jail, but how many they’d be sentenced to would be based on specifics of the case.