Keynesian economics is an economic philosophy developed by John Maynard Keynes. The idea behind Keynesian economics is that during times of recession and depression, the government should practice deficit spending. As a result, the government can take measures to stimulate the economy, as well as continue to provide social programs. Once the economy is doing well again, the debt can then be payed off.
Conservatives often point to deficit spending and label it as "fiscal irresponsibility". However, as long as there is not a net deficit over an economic cycle (as happened in Greece), Keynesian Economics is a rational and responsible system of managing the economy.