Net Income in business is the amount you get after subtracting all expenses from the revenue.
In accounting, it is calculated as Sales less Cost of goods sold, selling, general and administrative expenses, operating expenses, depreciation, taxes, interest on loans and other expenses. This number appears on a company's income statement and is an important measure of how profitable the company is.
For individuals, net income is just their total income (usually their salary) minus their total monthly expenses (rent, transport, food, clothing, school fees, entertainment)
It is considered extremely important for individuals to live within their means, which just means making sure their monthly expenses do not exceed total salary. This way, they don't have to borrow to cover basic monthly expenditure.