Brake fluid recovery in simple terms is when you heat the brake fluid to the highest possible temperature in actual driving conditions, it boils over and comes back to still allow some braking pedal feel.
Some brake fluids depending on formulation, once heated over its max temperature, the fluid will lose its ability to recover and significantly reduce its compressibility.
Pedal recovery will never be 100%, however, a good brake fluid recovery will still allow partial brake pedal pressure.