This is the 8493rd most frequent Russian word.
нижним
"Lower" or "bottom" (instrumental case of "нижний").
In this sentence, 'нижним' describes the lower floor, indicating its relative position in a multi-level structure.
Он показал нижним этажом старинный погреб.
He showed the lower floor's old cellar.
Here, 'нижним' is used to specify the shelf located at the bottom relative to the cabinet structure.
Книга лежит на нижнем полке шкафа.
The book is lying on the lower shelf of the cabinet.
In this example, 'нижним' describes a subdued or reduced level of lighting.