This basically means a fluke hit that an artist makes at the tail end of their relevance. In other words, the artist was plainly out of their "imperial phase" but they were still around and labels and radio programmers were still interested enough to keep trying. And when given the right song, there was still public good will to give them another chart-topper.

From 1960-1962 Gary U.S. Bonds scored 7 top 40 hits with 5 making the top 10. This would be referred to as his Imperial phase. He didn't chart again until 1981 with a cover of Bruce Springsteen's "This Little Girl" which reached #11.