Refers to a downward slope or a figurative decline. Can be used for physical slopes or for things like a downturn in fortune or activity.
この道は下坂が続いている。
This road continues downhill.
景気は下坂に向かっている。
The economy is on the decline.
Compound of 下 (しも, 'down') and 坂 (さか, 'slope'). The reading しもさか is a straightforward kun'yomi combination.