Earnings (also referred to as net income or net profit) measures how much money a company has earned from doing business over a given period of time (usually a year). Earnings shows what is left from the money a company has earned (revenue) after substracting all expenses.
In accounting terms it is often referred to as the "botton line" (because it is reported at the bottom of an income statement).
Net earnings = Total Revenue - Total Expenses (including operating expenses, financing costs and taxes)
This indicator will be included in my book: Key Performance Indicators - the 100 measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.