What is a black hole, exactly?

It’s an unbelievably dense object with a gravitational pull so strong that not even light can escape it. Anything that comes close enough to enter the gravitational field will be drawn towards it and eventually absorbed. Unfortunately, our understanding of physics breaks down at this point, and it’s beyond our knowledge what happens at the event horizon (the point of no return). Theoretically you could orbit the black hole if you’re travelling fast enough, but not once you pass the event horizon.
Black holes originate from supermassive collapsed stars, which then swallow matter (including other black holes), and they get more dense and more powerful with each swallowed object.
There you go. That’s the limit of my understanding at least. Scary things, black holes…