There are often said to be two basic issues in macroeconomics: business cycles and long-run growth. The argument in the lecture is that the gains to be had from further smoothing of the business cycle are quite small, while taxing and spending policies that promote growth would have a large, positive impact.
Macroeconomics was born as a distinct field in the 1940's, as a part of the intellectual response to the Great Depression. The term then referred to the body of knowledge and expertise that we hoped would prevent the recurrence of that economic disaster. My thesis in this lecture is that macroeconomics in this original sense has succeeded: Its central problem of depression prevention has been solved, for all practical purposes, and has in fact been solved for many decades. (p. 1; emphasis added)In fairness to Lucas, he was writing in 2002/3, when lots of people shared his notion that we already knew how to prevent depressions. But it's also fair to point out that in 2009 Lucas was arguing mockingly against efforts to apply what we knew about how to prevent a depression. We skirted the precipice after the financial meltdown of 2008, but it was no thanks to Lucas.