Neutrinos are detected all the time. We can even generate them in a particle accelerator and detect them from hundreds of miles away.
High-energy neutrinos, with energy tens of TeV (that’s higher energy than the LHC can produce) are rare, but they are still regularly observed.
What’s new is, when IceCube detected this high-energy neutrino, they were able to calculate the direction it came from, and relay it to other observatories around the world very quickly, to see if the source of the neutrino could be detected in some other way. And they did detect it - a blazar in the same part of the sky brightened around the same time. Which is not conclusive, but a pretty good indication that this blazar was the source of the high-energy neutrino.
The really exciting part is, this may indicate where ultra-high energy cosmic rays come from. Because that’s been a long-standing mystery in astrophysics. Although there are reasons to be cautious, as explained here.