Net Volume, or NV in short, is a cumulative volume indicator similar to OBV and A/D. Though it might look similar to these two indicators (especially A/D), rest assured it's better and more accurate than both. What it basically does, is dividing the volume session into buyers and sellers volumes; then subtracts former from the latter and adds the result to previous session's net volume (In the script, these two intermediate volumes are not calculated and only the subtraction is formulated in an integrated form).
Another important difference between NV and A/D indicator, is that it brings price gaps into account. That's why it requires next session's open price to calculate current session's NV. What it actually mean is that the price gap, in either direction, is the result of the traders' efforts in respective direction in previous session, as if the current session closes where next session opens.