It is intended to represent the total number of paid workers in the US, with the exception of farm, government and private-household employees, plus employees of non-profit organisations. The non-farm payrolls are typically released an hour before the official opening of the US stock market, (8.30am New York time) on the first Friday of each month, although the date will occasionally vary due to a public holiday.
As there are 24-hour sessions for many markets these days, reactions tend to be extremely fast. See our what are non-farm payrolls article to get a better understanding and learn why the payrolls report is particularly important in forex trading.