The Sixth Amendment guarantees that in "all criminal prosecutions, the accused shall enjoy the right to be informed of the nature and cause of the accusation." That means the government cannot just arrest you and figure out the charges later -- prosecutors must spell out exactly what crime you allegedly committed, with enough detail about the time, place, and circumstances for you to mount a defense.
The Supreme Court has enforced this requirement strictly. In Cole v. Arkansas (1948), the Court overturned convictions because the defendants were convicted under a different section of the statute than the one cited in the charges, meaning they never had a chance to defend against the actual accusations. In Russell v. United States (1962), the Court threw out contempt charges against witnesses who refused to answer a congressional subcommittee's questions because the indictment failed to identify the specific topic under investigation. Without that detail, the defendants could not know what they were accused of refusing to discuss. The notice requirement also protects against double jeopardy: if charges are specific enough, a defendant can later prove they already faced prosecution for the same offense. This right applies to every criminal case in America -- from a traffic violation in municipal court to a federal terrorism trial -- and it forces the government to show its hand before asking a jury to convict.