Hawking is referring to the Black hole information paradox.
In classical theories (such as Newtonian mechanics and General Relativity) the model is deterministic. If you know the precise initial conditions of a physical system you can predict its future evolution.
Quantum mechanics introduces indeterminacy: When a quantum system is observed, there is a probability it will be found in particular state. However, the quantum wavefunction evolves following deterministic laws, such as Schrodinger's equation. This means that if you know the wavefunction of a quantum system at any time, you can find its wavefunction at any future time. And with this wavefunction, you can calculate the probabilities of certain outcomes.
Black holes seem to violate this principle: The wavefunction of a quantum system that falls into a black hole is lost, as black holes only have properties of mass/energy, charge, and angular momentum. Moreover, when that mass/energy is returned to the universe by Hawking radiation, the wave function of the original matter is lost. The wave function of the radiation produced by Hawking radiation is indeterminate and and can't (even in principle) be determined from the wavefunction of the matter that collapsed to form the black hole.
Hawking initially suggested that information was indeed lost. However later in life he changed his mind, suggesting that wavefunction information is encoded at the event horizon, and Hawking radiation carries an imprint of the wavefunction of the matter that formed the black hole. This means that if you were (somehow) able to observe the wavefunction of the radiation given off by a black hole, you could calculate the wavefunction of the matter that formed the black hole...
In practical terms this doesn't do much for the astronaut that falls in. Wavefunctions are not generally observable (we can only infer facts about them from statistical observations of particles, and such observations cause the wavefunctions to collapse). Even if we could could observe wave functions, we couldn't observe them over the enormous lives of evaporating black holes (much much longer than the current age of the universe) and even if we did, we aren't certain how to work backwards to "break the code" of the information on the event horizon, nor reconstruct the astronaut from the wavefunction information that is collected this way.
So "Black holes ain't so black" the quantum information of the matter that falls into them isn't destroyed.