Gross sand is the thickness of the sand top to bottom.Net sand...

Gross sand is the thickness of the sand top to bottom.
Net sand is the number of meters in this sand which has porosity greater than a set cutoff (~7%).
Net pay is the number of meters of net sand that has movable oil in it.

For example: you have a 30m sand which contains some shale beds and some tight beds and ends up have 20 m of net sand. If there is an oil water contact in the sand and the top is oil and the bottom part of the sand is wet, you might oil have 10 m of net pay - and this is the most important number to estimate reserves and even flow rates. And all three numbers are mapped spatially to understand the reservoir, estimate reserves and spot new wells.