Paydirt is dirt forming the deposits that the miners mine for. Paydirt may form into paystreaks, or deposit into glory holes and hotspots. Placer deposits are pay gravel while hard rock deposits are ore.
Main pay is paydirt laid down in river beds from fast moving river channels, main pay would contain coarse gold. Side pay are laid down by slower creeks in creek beds that meander their channels, side pay would contain fine gold.
Top gravel is lower quality pay gravels lying above primary pay zones, which may or may not contain much gold.