Police received an anonymous tip that a black male standing at a particular bus stop wearing plaid shirt was carrying a gun. J.L. was a black male standing at that bus stop while wearing a plaid shirt, and there were two other people with him (only J.L. matched the suspect description, though). Nothing else was known about the reporting party.
Based on the anonymous tip, officers searched J.L. and recovered a handgun. They also searched the other two guys, apparently because it was physically possible to do so. J.L. was subsequently charged with CCW and with possessing a handgun whilst under 18. He moved to suppress the gun as the fruit of an unreasonable search, and this case made it all the way up to the Supreme Court.
The Court held that in order to establish reasonable suspicion, an anonymous tip must have sufficient indicia of reliability. The only such indication put forth in this decision is "the correct forecast of the subject's not easily predicted movements." The decision discussed another case where an RP predicted that a woman would leave a particular place in a particular kind of car and check into a particular hotel, and that she would have drugs. That case had been deemed by the court to just barely satisfy reasonable suspicion.
By contrast, the court held that the tip in J.L.'s case was not reliable enough to establish reasonable suspicion. The court recognized that the RP specifically described J.L., but held that simply providing enough information to identify a suspect doesn't make a tip reliable.
The prosecution had argued for a firearms exception that would make stops based on anonymous tips regarding guns reasonable per se. The court didn't like the slippery slope something like that would create, and talked about how if they were to humor that then eventually the exception would swallow the rule (because other kinds of criminals are known to carry guns, so anonymous tips not involving firearms could theoretically be treated like anonymous tips involving firearms...). So the Supreme Court specifically did not create a rule that would allow us to stop people based on anonymous tips regarding firearms.
The court did note that if the circumstances were different, such as if a case were to involve a suspect carrying a bomb, or if a case involved a place where the expectation of privacy is diminished (like an airport or school), then maybe it would be possible to justify a search using information that would otherwise be insufficient. But there was nothing like that here, so the evidence in J.L.'s case was suppressed.