Keynesian macroeconomics is named after John Maynard Keynes, who was the man who came up with the idea.
Keynesian economics came as a challenge to classical economics that was inspired by the Great Depression. Classical economics held that government should allow markets to do as they wished and that such markets would always bring the economy back to full employment. Keynes challenged this. He argued that governments might have to intervene in the economy to bring it back to full employment in times of deep recessions. He argued, for example, that governments would need to spend money (even if they had to go in debt to do so) as a way to increase aggregate demand and get an economy out of recession.
Keynesian economics is under challenge today from conservative economists who disagree with its tenets.