It indicates the direction of the airflow from the fans.
Your standard fan pulls air in from one direction and pushes it out the other.
Push means the fan is mounted/attached so that it is pushing air through something, e.g. the heatsink the case etc. Air is being pushed away from the fan into whatever it's meant to be cooling.
Pull means the fan is pulling air through something.
Usually you're trying to push the air in through the front of the case and pull it out the back of the case.
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