Hawking radiation predicts that black holes slowly lose mass and energy, eventually evaporating completely over incredibly long timescales. For stellar-mass black holes, this process takes far longer than the current age of the universe.
It matters because it challenges the idea that black holes are eternal.
It also opens discussions on the ultimate fate of matter and information in the universe.
Some black holes could completely vanish after trillions of years due to Hawking radiation.
[NASA, nasa.gov]