Long story short, using rap lyrics, Elonis posted on Facebook what some interpreted as threats of violence against them. Those ranged from his estranged wife to a FBI agent.
A federal jury convicted him. Essentially the charges were that he violated a federal law about communicating threats across state lines. He has already served three years of his 44-month sentence.
The legal questions, embedded in free speech, involve: Should intentionality be factored into a judgment about communication on the Internet and/or is it adequate to recognize that reasonable people felt threatened. The jury instructions in the original Elonis trial defined a "true threat," not protected by the First Amendment, as speech which reasonable people would interpret as portending harm.
In December the U.S. Supreme Court will be reviewing this case, now framed as "Elonis v. United States." As Dahlia Lithwick explains on Slate, the justices will have to bone up on a lot of material in which they probably have little expertise. That ranges from the role of Facebook in informal communications in current society to the ethos of rap music. Here is her article.
If SCOTUS confirms the lower court's jury decision, then those in habit of ranting negatively in public about other people in a threatening way will know that is not considered their right under the First Amendment. They could be arrested. If the rant was posted in social media, which is national therefore entails federal law, they could be charged with federal crimes, just as Elonis had been.
If Elonis gets off the hook, then it will be a free-for-all verbally on the Internet. The creative among us could position and package our threats as what could eventually wind up in winners of contests. Those competitions could be launched post-Elonis win to raise the bar on how we can do in our enemies. A good time could be ahead for those who didn't fare too well in anger management courses.