Volume refers to the amount of overall work you do during a session - how many sets and reps you perform. Intensity is the percentage of your 1RM (not specifically, but it refers to how much weight). For example, performing just a 1RM would be very high intensity, but low volume.
High volume and low intensity, on the other hand, would mean you're performing a whole bunch of sets and reps, but using a low percentage of your 1RM, one that allows you to perform said reps.
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