Stars live in a constant tug-of-war. They are large, so gravitational forces are huge. That makes them tend to collapse. The gravitational force is countered by what makes a star shine, which is constant explosions resulting from nuclear fusion. Equilibrium is reached between the inward pull of gravity and the outward push of explosions. In normal-sized stars, this equilibrium can be maintained for a very long time, possibly several billion years.
The process begins with fusion of hydrogen into helium. As the star ages, its hydrogen supply dwindles and new fusion processes begin to take prominence. These processes create heavier elements. When the star reaches the point where it is creating iron, the fusion process begins to lose to gravity. The process of creating iron by fusion requires more energy than it produces, so the star can no longer maintain its stability against the pull of gravity. The star begins to collapse.
For a star with a mass about 1.4 times that of our sun or less, it will collapse to the point where it becomes what is known as a white dwarf. It will be small and dim. For larger stars, especially much larger stars, something very strange can happen. It may collapse so fast and become so compact and dense that the escape velocity exceeds the speed of light. These objects emit no electromagnetic radiation (meaning no light) and hence the name “black hole”. Matter and energy can enter but nothing leaves.
Since we cannot see them, how can we know they are there? They may not give off any light, but they are massive and their gravitational influence on other bodies is easily seen. What are conditions like within a black hole? There are no theories for that, at least none that have any relation to the “regular” world. The laws of physics break down at the surface of the black hole, and we simply have nothing to compare the inside of the black hole with.