Impedance is resistance in an AC-circuit (Audio is an AC signal). It depends on inductance of all components in the signal path.
For example if your amplifier can deliver 7V peak voltage, it would push 220mA through a 32 Ω headphone or 45mA through a 150 Ω headphone.
I have a more detailed write up on impedance and power here:
What is wrong with unplugging headphones and plugging different ones in?
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