Something untimely has occurred prematurely, and it's usually not good news. If you hear about someone's untimely death, it must mean it was an unexpected death.
When something is timely, it happens at the right time. When something is untimely, it means the opposite: it happened at an unfortunate or unexpected time. Usually, this means it happened much too early. The most common use of this term is probably in discussing someone’s untimely death, meaning they died young. Other disasters that happen to young people might be described as untimely as well.