If Net Profit Margin increases over time:
An increasing Net Profit Margin usually shows the net profit out of each dollar of sales has become larger.
If Net Profit Margin decreases over time:
A decreasing Net Profit Margin usually shows the net profit out of each dollar of sales has become smaller.
If Net Profit Margin stays the same over time:
An unchanged Net Profit Margin usually shows the net profit out of each dollar of sales has remained the same.