Something that threatens makes its danger known. Storm clouds threaten rain, and bullies threaten other kids.
Unfortunately, there are lots of people and things that threaten — it's a dangerous world. If someone threatens you with violence, you should call the police. The police may then threaten the criminal with arresting them. Nations threaten each other before going to war. You can also say a dangerous situation is threatening, like gloomy weather. Sometimes this word is used more lightheartedly, like if an old friend threatens to hug you to death.