noun
long gentle slope
Describes a slope that is not steep but continues for a long distance, often tiring to walk or cycle up. The だらだら part suggests a tedious, drawn-out quality.
この道はずっとだらだら坂が続いていて、自転車だと結構きつい。
This road is a long, gentle slope all the way, and it's pretty tough on a bicycle.
Compound of the mimetic adverb だらだら (dripping, dragging on) and 坂 (slope). The exact origin of だらだら is uncertain, but it is sound-symbolic for something prolonged and monotonous.