Why An ideal operational amplifier has infinite input impedance, infinite open-loop gain, and zero output impedance. Real devices come close: input impedance is in the megohms or higher, and output impedance is typically a fraction of an ohm to a few ohms, especially once negative feedback is applied. A very low output impedance means the output voltage barely sags when the op-amp drives a load, so the stage acts like an ideal voltage source.
Watch out Very high describes the op-amp's input impedance, not its output. The specific values like 100 ohms or 10,000 ohms are too large for an op-amp output and just look plausible.
Op-amp ideal: input impedance infinite, output impedance zero, gain infinite. High in, low out.