A black hole is a region of spacetime exhibiting gravity so strong that nothing can escape - not even energy. Black holes are speculated to exist for a number of reasons, the most common being the collapse of a massive star.
Most of the black holes in Outer Wilds are created by the Nomai. The Black Hole Forge near the natural black hole at the center of Brittle Hollow was where these black holes were created in order to craft warp cores.
A central conceit to the game is that of warp travel using black and white holes - with the black holes accepting all matter, and the white holes rejecting them. As a converse of the black holes, nothing can enter a white hole.