A black hole can be the compact object left when a massive star collapses and its core is not stopped by neutron pressure.
When a sufficiently massive star exhausts its nuclear fuel, its core loses the pressure that supported it against gravity. The core may collapse catastrophically, sometimes producing a supernova and sometimes collapsing with little visible explosion. If the remaining core becomes compact enough, an event horizon forms and a black hole results.
Not every massive star creates a black hole. The outcome depends on the star’s initial mass, metallicity, mass loss, rotation, binary interactions, and the details of the explosion. Some collapses leave neutron stars instead, whose matter is supported by nuclear forces and neutron degeneracy pressure.
The black hole itself is not a solid remnant with a conventional surface. The event horizon marks a causal boundary, while the collapsed matter’s ultimate interior description requires physics that combines gravity with quantum theory.