Net margin is what is left of every unit of sales after all costs, interest, and taxes are paid. It is the most widely quoted profitability figure, the true bottom line.
A net margin above 20% is exceptional and usually points to a strong, lasting advantage that stops rivals from competing the profit away, but plenty of excellent businesses sit well below that. The level depends heavily on the type of business: a software company keeps most of each sale because copies cost almost nothing to produce, while a supermarket runs on razor-thin margins and makes its money on sheer volume. Comparing those two on net margin tells you nothing useful.
Always judge a company against others in its own industry, and average the figure over about five years, since any single year gets thrown off by one-time items and the timing of taxes.