The no-hair theorem says that a stationary black hole is completely described by mass, electric charge, and angular momentum.
The phrase “no hair” means that a black hole cannot retain most of the detailed information about the matter that formed it. Two black holes with different histories can look identical from outside if these three measurable properties match. In simplified astrophysical discussions, electric charge is usually negligible, leaving mass and spin as the dominant characteristics.
The theorem is not one single result proved in every possible situation. It summarizes several mathematical uniqueness results under specific assumptions, including general relativity and suitable conditions on the surrounding spacetime. Real black holes may also be surrounded by magnetic fields, gas, stars, and disks, but those surroundings are not intrinsic hair.
The idea is closely linked to the black-hole information problem. Quantum theory appears to preserve information, while classical black-hole descriptions hide almost all formation details behind a few numbers.