Mostly for non-macroeconomists.
I first learned macroeconomics in the very early 1970's in the UK. I learned that the macroeconomy was not automatically self-equilibrating, and that the government should use fiscal policy to target "full employment" (aka "potential output"). The government should loosen fiscal policy when the economy was below potential and tighten fiscal policy when the economy was above potential. We didn't pay much attention to monetary policy. And we didn't pay much attention to inflation either, at least in the models, though it was getting harder and harder to ignore in reality. I think we hoped for what we would nowadays call "Divine Coincidence": that if we were successful at targeting "full employment" then inflation would take care of itself. And if it didn't, then maybe we needed some additional policy lever, like wage and price controls.
Let's call that "Old Keynesian" fiscal policy. Many people will still think of "Keynesian" fiscal policy as something like that.
But times change.