Translation needed on CFA Wording - Bonds Because, english, through, search

trogulj

Hi,
Can anyone explain what this means in “plain english”.
Taken from Reading 30, Book 4, page 79…
“Because new 15-year structures take 5 years to descend along a positively sloped yield curve to their underlying 10-year bellwether, 15-year maturities hold less appeal for many investors in search of return through price appreciation emanating from benchmark rolldown”
Benchmark rolldown? I just don’t know what this means in english?