This is the 3102nd most frequent Japanese word.
低く
"Low" or "short" (in height or depth).
In this sentence, "低く" is used as an adverb to describe the nature of his voice, implying its tonal pitch being low.
彼の声はとても低くて、聞き取りにくかった。
His voice was very low, and it was difficult to hear.
Here, "低く" is describing the physical height of the desk as being low.
この机はかなり低くて、子供用だ。
This desk is quite low and is meant for children.
In this instance, "低く" expresses the low aspect of the temperature, associated with cold weather conditions.