Most lawns need 1-1.5 inches of water per week, either from rainfall or irrigation. Water deeply and less frequently to encourage deep root growth.
N-P-K stands for Nitrogen (leaf growth), Phosphorus (root/flower development), and Potassium (overall health). The numbers show the percentage of each.
2-4 inches is ideal for most gardens. Too little won't suppress weeds effectively, too much can suffocate plant roots and promote disease.