This is the 7227th most frequent Japanese word.
下り坂
"Downhill slope" or "decline."
Here, '下り坂' is used literally to describe a steeply sloped road.
この道は急な下り坂です。
This road is a steep downhill slope.
In this context, '下り坂' is used metaphorically to describe the declining performance in grades.
成績が下り坂になっています。
Grades are declining.
Here, '下り坂' implies worsening weather conditions and is used metaphorically.