So, there was a man named Elonis who was angry at some people and he wrote some really mean things about them on the internet. The government didn't like what Elonis wrote and they said he broke a law that says you can't threaten someone with violence.
Elonis said that he didn't really mean to hurt anyone and just wrote the things because he was angry. But the government didn't believe him and they said he still broke the law even if he didn't really want to hurt anyone.
So, they took him to court and the court had to decide whether what Elonis wrote counted as a threat or not. Some people thought it did and some people thought it didn't.
In the end, the Supreme Court (the highest court in the land) had to decide. They said that just because someone says something mean or scary online, it doesn't automatically mean they're making a threat. The court said that to break the law, a person has to actually mean to threaten someone with violence.
That's basically what happened in Elonis v. United States. It was all about whether what someone writes on the internet is a real threat or just free speech.