The announcement was really about a week ago, but I have not gotten around to posting about this until now. Edmund Phelps is a macroeconomist, so his accomplishments will not mean as much until next semester.
His main contribution is the idea of a natural rate of unemployment, which is a certain level of unemployment that will exist even when the economy is functioning at its potential. He also wrote about the importance of inflationary expectations -- the fact that it matters what people think inflation is going to be in the future.
Phelps is honestly not an economist whose work I know very well, but here is the press release announcing what he got the award for, and here is a summary of his accomplishments by economist Tyler Cowen.