Some stars are so big that when they burn up all their fuel they end their life with a bang. The explode in what is called a supernova.
Sometimes these stars are so big (we’re talking many many times heavier than our own sun) that all the stuff they are made from collapses to a single point. This point is the black hole. The gravity of a black hole is so strong that not even light can escape, hence the name black. The event horizon describes area around the black hole beyond which light cannot escape.
But here’s the problem. Light gives us information. But light cannot escape the event horizon. And at the moment there is no other way of learning what is inside a black hole without using light. So unfortunately we can’t look past an event horizon.