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.
The black holes in Outer Wilds all seem to be of artificial manufacture. Brittle Hollow's central black hole sits beneath the Black Hole Forge, and it's implied that it was created by the Nomai millennia before the player encounters the planet.
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.